Kumena's Speaker

Kumena's Speaker gets +1/+1 as long as you control another Merfolk or an Island.

Deeproot Elite

Whenever another Merfolk you control enters, put a +1/+1 counter on target Merfolk you control.

Stonybrook Schoolmaster

Whenever Stonybrook Schoolmaster becomes tapped, you may create a 1/1 blue Merfolk Wizard creature token.

Stonybrook Banneret

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Merfolk spells and Wizard spells you cast cost {1} less to cast.

Forest

Universal Automaton

Changeling (This card is every creature type.)

Lord of Atlantis

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

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.)

Kira, Great Glass-Spinner

Flying
Creatures you control have "Whenever this creature becomes the target of a spell or ability for the first time each turn, counter that spell or ability."

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}.

Forerunner of the Heralds

When Forerunner of the Heralds enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.
Whenever another Merfolk you control enters, put a +1/+1 counter on Forerunner of the Heralds.

Merrow Commerce

At the beginning of your end step, untap all Merfolk you control.

Crib Swap

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

Merfolk Branchwalker

When Merfolk Branchwalker enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

Akroma's Memorial

Creatures you control have flying, first strike, vigilance, trample, haste, and protection from black and from red.

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.

Sigil Tracer

{1}{U}, Tap two untapped Wizards you control: Copy target instant or sorcery spell. You may choose new targets for the copy.

Jade Bearer

When Jade Bearer enters, put a +1/+1 counter on another target Merfolk you control.

Kumena, Tyrant of Orazca

Tap another untapped Merfolk you control: Kumena, Tyrant of Orazca can't be blocked this turn.
Tap three untapped Merfolk you control: Draw a card.
Tap five untapped Merfolk you control: Put a +1/+1 counter on each Merfolk you control.

Earthcraft

Tap an untapped creature you control: Untap target basic land.

Merfolk Sovereign

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

Deeproot Champion

Whenever you cast a noncreature spell, put a +1/+1 counter on Deeproot Champion.

Jadelight Ranger

When Jadelight Ranger enters, it explores, then it explores again. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard. Then repeat this process.)

Chulane, Teller of Tales

Vigilance
Whenever you cast a creature spell, draw a card, then you may put a land card from your hand onto the battlefield.
{3}, {T}: Return target creature you control to its owner's hand.

Intruder Alarm

Creatures don't untap during their controllers' untap steps.
Whenever a creature enters, untap all creatures.

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.

Herald of Secret Streams

Creatures you control with +1/+1 counters on them can't be blocked.

Harbinger of the Tides

You may cast Harbinger of the Tides as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
When Harbinger of the Tides enters, you may return target tapped creature an opponent controls to its owner's hand.

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.)

Lullmage Mentor

Whenever a spell or ability you control counters a spell, you may create a 1/1 blue Merfolk creature token.
Tap seven untapped Merfolk you control: Counter target spell.

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.

Birthing Boughs

{4}, {T}: Create a 2/2 colorless Shapeshifter creature token with changeling. (It has every creature type.)

Cursecatcher

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

Wistful Selkie

Merrow Reejerey

Other Merfolk creatures you control get +1/+1.
Whenever you cast a Merfolk spell, you may tap or untap target permanent.

Cryptolith Rite

Creatures you control have "{T}: Add one mana of any color."

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.

Tatyova, Benthic Druid

Landfall — Whenever a land you control enters, you gain 1 life and draw a card.