Saltwater Stalwart

Whenever Saltwater Stalwart deals damage to an opponent, target player draws a card.

Have

Deeproot Waters

Whenever you cast a Merfolk spell, create a 1/1 blue Merfolk creature token with hexproof. (A creature with hexproof can't be the target of spells or abilities your opponents control.)

True-Name Nemesis

As True-Name Nemesis enters, choose a player.
True-Name Nemesis has protection from the chosen player.

Crib Swap

Changeling (This card is every creature type.)
Exile target creature. Its controller creates a 1/1 colorless Shapeshifter creature token with changeling.

Deeptread Merrow

{U}: Deeptread Merrow gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)

Blackblade Reforged

Equipped creature gets +1/+1 for each land you control.
Equip legendary creature {3}
Equip {7}

Unsettled Mariner

Changeling (This card is every creature type.)
Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays {1}.

An Offer You Can't Refuse

Counter target noncreature spell. Its controller creates two Treasure tokens. (They're artifacts with "{T}, Sacrifice this artifact: Add one mana of any color.")

Steel of the Godhead

Enchant creature
As long as enchanted creature is white, it gets +1/+1 and has lifelink. (Damage dealt by the creature also causes its controller to gain that much life.)
As long as enchanted creature is blue, it gets +1/+1 and can't be blocked.

Kindred Discovery

As Kindred Discovery enters, choose a creature type.
Whenever a creature you control of the chosen type enters or attacks, draw a card.

Vodalian Hexcatcher

Flash
Other Merfolk you control get +1/+1.
Sacrifice a Merfolk: Counter target noncreature spell unless its controller pays {1}.

Svyelun of Sea and Sky

Svyelun of Sea and Sky has indestructible as long as you control at least two other Merfolk.
Whenever Svyelun attacks, draw a card.
Other Merfolk you control have ward {1}. (Whenever another Merfolk you control becomes the target of a spell or ability an opponent controls, counter it unless that player pays {1}.)

Silvergill Adept

As an additional cost to cast this spell, reveal a Merfolk card from your hand or pay {3}.
When Silvergill Adept enters, draw a card.

Argentum Armor

Equipped creature gets +6/+6.
Whenever equipped creature attacks, destroy target permanent.
Equip {6}

Kopala, Warden of Waves

Spells your opponents cast that target a Merfolk you control cost {2} more to cast.
Abilities your opponents activate that target a Merfolk you control cost {2} more to activate.

Stoneforge Masterwork

Equipped creature gets +1/+1 for each other creature you control that shares a creature type with it.
Equip {2}

Remove? 

Glacial Floodplain

({T}: Add {W} or {U}.)
Glacial Floodplain enters tapped.

Haunted Cloak

Equipped creature has vigilance, trample, and haste.
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Sol Ring

River Sneak

River Sneak can't be blocked.
Whenever another Merfolk you control enters, River Sneak gets +1/+1 until end of turn.

Negate

Tempest Caller

When Tempest Caller enters, tap all creatures target opponent controls.

Afterlife

Destroy target creature. It can't be regenerated. Its controller creates a 1/1 white Spirit creature token with flying.

Tidebinder Mage

When Tidebinder Mage enters, tap target red or green creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control Tidebinder Mage.

Star Compass

Star Compass enters tapped.
{T}: Add one mana of any color that a basic land you control could produce.

Sygg, River Guide

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
{1}{W}: Target Merfolk you control gains protection from the color of your choice until end of turn.

Cursecatcher

Sacrifice Cursecatcher: Counter target instant or sorcery spell unless its controller pays {1}.

Port Town

As Port Town enters, you may reveal a Plains or Island card from your hand. If you don't, Port Town enters tapped.
{T}: Add {W} or {U}.

Sigil of Sleep

Enchant creature
Whenever enchanted creature deals damage to a player, return target creature that player controls to its owner's hand.

Aquitect's Will

Put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it. If you control a Merfolk, draw a card.

Esper Sentinel

Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays {X}, where X is Esper Sentinel's power.

Path to Exile

Exile target creature. Its controller may search their library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Tideshaper Mystic

{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.

Temple of the False God

{T}: Add {C}{C}. Activate only if you control five or more lands.

Waterspout Weavers

Kinship — At the beginning of your upkeep, you may look at the top card of your library. If it shares a creature type with Waterspout Weavers, you may reveal it. If you do, each creature you control gains flying until end of turn.

Unbreakable Formation

Creatures you control gain indestructible until end of turn.
Addendum — If you cast this spell during your main phase, put a +1/+1 counter on each of those creatures and they gain vigilance until end of turn.

Distant Melody

Choose a creature type. Draw a card for each permanent you control of that type.

Military Intelligence

Whenever you attack with two or more creatures, draw a card.

Annul

Counter target artifact or enchantment spell.

Azorius Chancery

Azorius Chancery enters tapped.
When Azorius Chancery enters, return a land you control to its owner's hand.
{T}: Add {W}{U}.

Make a Stand

Creatures you control get +1/+0 and gain indestructible until end of turn. (Damage and effects that say "destroy" don't destroy them.)

Tazeem Roilmage

Kicker {4} (You may pay an additional {4} as you cast this spell.)
When Tazeem Roilmage enters, if it was kicked, return target instant or sorcery card from your graveyard to your hand.

Swords to Plowshares

Exile target creature. Its controller gains life equal to its power.

Myriad Landscape

Myriad Landscape enters tapped.
{T}: Add {C}.
{2}, {T}, Sacrifice Myriad Landscape: Search your library for up to two basic land cards that share a land type, put them onto the battlefield tapped, then shuffle.

Tide Shaper

Kicker {1} (You may pay an additional {1} as you cast this spell.)
When Tide Shaper enters, if it was kicked, target land becomes an Island for as long as Tide Shaper remains on the battlefield.
Tide Shaper gets +1/+1 as long as an opponent controls an Island.

Tidal Warrior

{T}: Target land becomes an Island until end of turn.

Scroll Thief

Whenever Scroll Thief deals combat damage to a player, draw a card.

Bident of Thassa

Whenever a creature you control deals combat damage to a player, you may draw a card.
{1}{U}, {T}: Creatures your opponents control attack this turn if able.

Master of the Pearl Trident

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Ojutai Monument

{T}: Add {W} or {U}.
{4}{W}{U}: Ojutai Monument becomes a 4/4 white and blue Dragon artifact creature with flying until end of turn.

Harpoon Sniper

{W}, {T}: Harpoon Sniper deals X damage to target attacking or blocking creature, where X is the number of Merfolk you control.

Awesome Presence

Enchant creature
Enchanted creature can't be blocked unless defending player pays {3} for each creature they control that's blocking it.

Goldvein Pick

Equipped creature gets +1/+1.
Whenever equipped creature deals combat damage to a player, create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")
Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Cleric of Chill Depths

Whenever Cleric of Chill Depths blocks a creature, that creature doesn't untap during its controller's next untap step.

Command Tower

{T}: Add one mana of any color in your commander's color identity.

Throne of the God-Pharaoh

At the beginning of your end step, each opponent loses life equal to the number of tapped creatures you control.

Dismantling Blow

Kicker {2}{U} (You may pay an additional {2}{U} as you cast this spell.)
Destroy target artifact or enchantment. If this spell was kicked, draw two cards.

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.

Lord of Atlantis

Other Merfolk get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Island

Wanderwine Prophets

Champion a Merfolk (When this enters, sacrifice it unless you exile another Merfolk you control. When this leaves the battlefield, that card returns to the battlefield.)
Whenever Wanderwine Prophets deals combat damage to a player, you may sacrifice a Merfolk. If you do, take an extra turn after this one.

Crush Contraband

Choose one or both —
• Exile target artifact.
• Exile target enchantment.

Staggering Insight

Enchant creature
Enchanted creature gets +1/+1 and has lifelink and "Whenever this creature deals combat damage to a player, draw a card."

Thieving Skydiver

Kicker {X}. X can't be 0. (You may pay an additional {X} as you cast this spell.)
Flying
When Thieving Skydiver enters, if it was kicked, gain control of target artifact with mana value X or less. If that artifact is an Equipment, attach it to Thieving Skydiver.

Citywide Bust

Destroy all creatures with toughness 4 or greater.

Sword of Vengeance

Equipped creature gets +2/+0 and has first strike, vigilance, trample, and haste.
Equip {3}

Plains

Folk Hero

Commander creatures you own have "Whenever you cast a spell that shares a creature type with this creature, draw a card. This ability triggers only once each turn."

Veteran of the Depths

Whenever Veteran of the Depths becomes tapped, you may put a +1/+1 counter on it.

Marble Diamond

Marble Diamond enters tapped.
{T}: Add {W}.

Fishliver Oil

Enchant creature (Target a creature as you cast this. This card enters attached to that creature.)
Enchanted creature has islandwalk. (It can't be blocked as long as defending player controls an Island.)

Enclave Elite

Multikicker {1}{U} (You may pay an additional {1}{U} any number of times as you cast this spell.)
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Enclave Elite enters with a +1/+1 counter on it for each time it was kicked.

Sejiri Refuge

Sejiri Refuge enters tapped.
When Sejiri Refuge enters, you gain 1 life.
{T}: Add {W} or {U}.

Arcane Signet

Seafloor Oracle

Whenever a Merfolk you control deals combat damage to a player, draw a card.

Streambed Aquitects

{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.

Merfolk Looter

Prismatic Lens

{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Curiosity

Enchant creature
Whenever enchanted creature deals damage to an opponent, you may draw a card.

Robe of the Archmagi

Whenever equipped creature deals combat damage to a player, you draw that many cards.
Equip {4}
Equip Shaman, Warlock, or Wizard {1}

Mist-Cloaked Herald

Curse of the Swine

Exile X target creatures. For each creature exiled this way, its controller creates a 2/2 green Boar creature token.

Coralhelm Commander

Level up {1} ({1}: Put a level counter on this. Level up only as a sorcery.)
LEVEL 2-3
3/3
Flying
LEVEL 4+
4/4
Flying
Other Merfolk creatures you control get +1/+1.

Drowner of Secrets

Tap an untapped Merfolk you control: Target player mills a card.

Chart a Course

Draw two cards. Then discard a card unless you attacked this turn.

Coastal Tower

Coastal Tower enters tapped.
{T}: Add {W} or {U}.

Spreading Seas

Enchant land
When Spreading Seas enters, draw a card.
Enchanted land is an Island.

Azorius Guildgate

Azorius Guildgate enters tapped.
{T}: Add {W} or {U}.

Merrow Harbinger

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Merrow Harbinger enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.

Azorius Locket

{T}: Add {W} or {U}.
{W/U}{W/U}{W/U}{W/U}, {T}, Sacrifice Azorius Locket: Draw two cards.

Wake Thrasher

Whenever a permanent you control becomes untapped, Wake Thrasher gets +1/+1 until end of turn.

Inkfathom Divers

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Inkfathom Divers enters, look at the top four cards of your library, then put them back in any order.

Azorius Signet

Sage's Dousing

Counter target spell unless its controller pays {3}. If you control a Wizard, draw a card.

Brainstorm

Draw three cards, then put two cards from your hand on top of your library in any order.

Decanter of Endless Water

You have no maximum hand size.
{T}: Add one mana of any color.

Ravenform

Exile target artifact or creature. Its controller creates a 1/1 blue Bird creature token with flying.
Foretell {U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Merrow Levitator

{T}: Target creature gains flying until end of turn.
Whenever you cast a blue spell, you may untap Merrow Levitator.

Reflections of Littjara

As Reflections of Littjara enters, choose a creature type.
Whenever you cast a spell of the chosen type, copy that spell. (A copy of a permanent spell becomes a token.)

Silvergill Douser

{T}: Target creature gets -X/-0 until end of turn, where X is the number of Merfolk and/or Faeries you control.

Meandering River

Meandering River enters tapped.
{T}: Add {W} or {U}.

Fallowsage

Whenever Fallowsage becomes tapped, you may draw a card.

Merfolk Sovereign

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.

Judge of Currents

Whenever a Merfolk you control becomes tapped, you may gain 1 life.