Ancient Crab

Flying Dolphin-Fish

Flying (This creature can't be blocked except by creatures with flying or reach.)

The Reality Chip

You may look at the top card of your library any time.
As long as The Reality Chip is attached to a creature, you may play lands and cast spells from the top of your library.
Reconfigure {2}{U} ({2}{U}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Turtle-Seals

Vigilance (Attacking doesn't cause this creature to tap.)

Coral Eel

Lagoon Breach

The owner of target attacking creature you don't control puts it on their choice of the top or bottom of their library.

Horned Loch-Whale

Flash
Ward {2}
This creature enters tapped unless it's your turn.

Horned Loch-Whale // Lagoon Breach

Colossal Whale

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever this creature attacks, you may exile target creature defending player controls until this creature leaves the battlefield. (That creature returns under its owner's control.)

Purple-Crystal Crab

Elusive Krasis

This creature can't be blocked.
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Crocanura

Reach (This creature can block creatures with flying.)
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Hydroid Krasis

When you cast this spell, you gain half X life and draw half X cards. Round down each time.
Flying, trample
This creature enters with X +1/+1 counters on it.

Bounding Krasis

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, you may tap or untap target creature.

Hedron Crab

Landfall — Whenever a land you control enters, target player mills three cards. (They put the top three cards of their library into their graveyard.)

Fecund Greenshell

Reach
As long as you control ten or more lands, creatures you control get +2/+2.
Whenever this creature or another creature you control with toughness greater than its power enters, look at the top card of your library. If it's a land card, you may put it onto the battlefield tapped. Otherwise, put it into your hand.

Harpoon Sniper

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

Tui and La, Moon and Ocean

Whenever Tui and La become tapped, draw a card.
Whenever Tui and La become untapped, put a +1/+1 counter on them.

Colossal Skyturtle

Flying, ward {2}
Channel — {2}{G}, Discard this card: Return target card from your graveyard to your hand.
Channel — {1}{U}, Discard this card: Return target creature to its owner's hand.

Bedrock Tortoise

During your turn, creatures you control have hexproof.
Each creature you control with toughness greater than its power assigns combat damage equal to its toughness rather than its power.

Skyreach Manta

Sunburst (This creature enters with a +1/+1 counter on it for each color of mana spent to cast it.)
Flying

Horseshoe Crab

Morska, Undersea Sleuth

You have no maximum hand size.
At the beginning of your upkeep, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this token: Draw a card.")
Whenever you draw your second card each turn, put two +1/+1 counters on Morska.

Chasm Skulker

Whenever you draw a card, put a +1/+1 counter on this creature.
When this creature dies, create X 1/1 blue Squid creature tokens with islandwalk, where X is the number of +1/+1 counters on this creature. (They can't be blocked as long as defending player controls an Island.)

Ruin Crab

Landfall — Whenever a land you control enters, each opponent mills three cards. (To mill a card, a player puts the top card of their library into their graveyard.)

Charix, the Raging Isle

Spells your opponents cast that target Charix cost {2} more to cast.
{3}: Charix gets +X/-X until end of turn, where X is the number of Islands you control.

Encumbered Reejerey

This creature enters with three -1/-1 counters on it.
Whenever this creature becomes tapped while it has a -1/-1 counter on it, remove a -1/-1 counter from it.

Scuttlegator

Defender
{6}{G/U}{G/U}: Adapt 3. (If this creature has no +1/+1 counters on it, put three +1/+1 counters on it.)
As long as this creature has a +1/+1 counter on it, it can attack as though it didn't have defender.

Great Whale

When this creature enters, untap up to seven lands.

Wanderbrine Trapper

{1}, {T}, Tap another untapped creature you control: Tap target creature an opponent controls.

Sharktocrab

{2}{G}{U}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.)
Whenever one or more +1/+1 counters are put on this creature, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Deepglow Skate

When this creature enters, double the number of each kind of counter on any number of target permanents.

Shore Lurker

Flying
When this creature enters, surveil 1. (Look at the top card of your library. You may put it into your graveyard.)

Giant Koi

Waterbend {3}: This creature can't be blocked this turn. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for {1}.)
Islandcycling {2} ({2}, Discard this card: Search your library for an Island card, reveal it, put it into your hand, then shuffle.)

Ancient Adamantoise

Vigilance, ward {3}
Damage isn't removed from this creature during cleanup steps.
All damage that would be dealt to you and other permanents you control is dealt to this creature instead.
When this creature dies, exile it and create ten tapped Treasure tokens.

Crocodile of the Crossing

Haste
When this creature enters, put a -1/-1 counter on target creature you control.

Fleet Swallower

Whenever this creature attacks, target player mills half their library, rounded up.

Judge of Currents

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

Adept Watershaper

Other tapped creatures you control have indestructible.

Star Whale

Flying, vigilance
Other creatures you control have ward {2}.
Suspend 6—{1}{U} (Rather than cast this card from your hand, you may pay {1}{U} and exile it with six time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost. It has haste.)

Sigiled Starfish

{T}: Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Stonybrook Schoolmaster

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

Aethertide Whale

Flying
When this creature enters, you get six {E} (energy counters).
Pay {E}{E}{E}{E}: Return this creature to its owner's hand.

Flumph

Defender, flying
Whenever this creature is dealt damage, you and target opponent each draw a card.

Eel-Hounds

Trample (This creature can deal excess combat damage to the player it's attacking.)
Whenever this creature attacks, another target creature you control gets +2/+2 and gains trample until end of turn.

Glowing Anemone

When this creature enters, you may return target land to its owner's hand.

Bubble Smuggler

Disguise {5}{U} (You may cast this card face down for {3} as a 2/2 creature with ward {2}. Turn it face up any time for its disguise cost.)
As this creature is turned face up, put four +1/+1 counters on it.