Have

Tideforce Elemental

{U}, {T}: You may tap or untap another target creature.
Landfall — Whenever a land you control enters, you may untap this creature.

Best

Coral Merfolk

Getting

Flow of Ideas

Merfolk Observer

When this creature enters, look at the top card of target player's library.

Eel Umbra

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature gets +1/+1.
Umbra armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Into the Roil

Kicker {1}{U} (You may pay an additional {1}{U} as you cast this spell.)
Return target nonland permanent to its owner's hand. If this spell was kicked, draw a card.

Kraken's Eye

Whenever a player casts a blue spell, you may gain 1 life.

Essence Scatter

Glimmerbell

Mystic Skyfish

Whenever you draw your second card each turn, this creature gains flying until end of turn.

Clinging Anemones

Defender
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.)

Horned Turtle

Living Tsunami

Flying
At the beginning of your upkeep, sacrifice this creature unless you return a land you control to its owner's hand.

Giant Tortoise

This creature gets +0/+3 as long as it's untapped.

Shoal Serpent

Defender
Landfall — Whenever a land you control enters, this creature loses defender until end of turn.

Merfolk

Negate

Dreadwaters

Target player mills X cards, where X is the number of lands you control.

Merfolk Mesmerist

Simic Fluxmage

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.)
{1}{U}, {T}: Move a +1/+1 counter from this creature onto target creature.

Mind Spring

Merfolk Looter

Dispel

Surestrike Trident

Equipped creature has first strike and "{T}, Unattach Surestrike Trident: This creature deals damage equal to its power to target player or planeswalker."
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery. This card enters unattached and stays on the battlefield if the creature leaves.)

Ethereal Whiskergill

Flying
This creature can't attack unless defending player controls an Island.

Vapor Snare

Enchant creature
You control enchanted creature.
At the beginning of your upkeep, sacrifice this Aura unless you return a land you control to its owner's hand.

Constricting Tendrils

Target creature gets -3/-0 until end of turn.
Cycling {2} ({2}, Discard this card: Draw a card.)

Don't Have

Cosi's Trickster

Whenever an opponent shuffles their library, you may put a +1/+1 counter on this creature.

Merfolk Skyscout

Flying
Whenever this creature attacks or blocks, untap target permanent.

Coral Barrier

Defender (This creature can't attack.)
When this creature enters, create a 1/1 blue Squid creature token with islandwalk. (It can't be blocked as long as defending player controls an Island.)

Retreat to Coralhelm

Landfall — Whenever a land you control enters, choose one —
• You may tap or untap target creature.
• Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Narcolepsy

Enchant creature
At the beginning of each upkeep, if enchanted creature is untapped, tap it.

Lethargy Trap

If three or more creatures are attacking, you may pay {U} rather than pay this spell's mana cost.
Attacking creatures get -3/-0 until end of turn.

Protective Bubble

Enchant creature
Enchanted creature can't be blocked and has shroud. (It can't be the target of spells or abilities.)

Rite of Replication

Kicker {5} (You may pay an additional {5} as you cast this spell.)
Create a token that's a copy of target creature. If this spell was kicked, create five of those tokens instead.

Stonybrook Angler

{1}{U}, {T}: You may tap or untap target creature.

Armored Whirl Turtle

Merrow Levitator

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

Sleep

Tap all creatures target player controls. Those creatures don't untap during that player's next untap step.

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.

Triton Shorethief

Merfolk of the Pearl Trident

Tricks of the Trade

Enchant creature
Enchanted creature gets +2/+0 and can't be blocked.

Merfolk Sovereign

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

Copy

(This token can be used to represent a token that's a copy of a permanent.)

Domestication

Enchant creature
You control enchanted creature.
At the beginning of your end step, if enchanted creature's power is 4 or greater, sacrifice this Aura.

Calcite Snapper

Shroud (This creature can't be the target of spells or abilities.)
Landfall — Whenever a land you control enters, you may switch this creature's power and toughness until end of turn.

Treasure Hunt

Reveal cards from the top of your library until you reveal a nonland card, then put all cards revealed this way into your hand.

Benthicore

When this creature enters, create two 1/1 blue Merfolk Wizard creature tokens.
Tap two untapped Merfolk you control: Untap this creature. It gains shroud until end of turn. (It can't be the target of spells or abilities.)

Zephyr Net

Enchant creature
Enchanted creature has defender and flying.

Triton Tactics

Up to two target creatures each get +0/+3 until end of turn. Untap those creatures. At this turn's next end of combat, tap each creature that was blocked by one of those creatures this turn and it doesn't untap during its controller's next untap step.

Cold-Water Snapper

Hexproof (This creature can't be the target of spells or abilities your opponents control.)

Jwari Scuttler

Breaching Hippocamp

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

Caller of Gales

{1}{U}, {T}: Target creature gains flying until end of turn.

Maritime Guard

Bewilder

Target creature gets -3/-0 until end of turn.
Draw a card.

Tazeem Roilmage

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

Tidebinder Mage

When this creature 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 this creature.

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.)
This creature enters with a +1/+1 counter on it for each time it was kicked.

Wavebreak Hippocamp

Whenever you cast your first spell during each opponent's turn, draw a card.

Drowner of Secrets

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

Merfolk Wayfinder

Flying
When this creature enters, reveal the top three cards of your library. Put all Island cards revealed this way into your hand and the rest on the bottom of your library in any order.

Skyclave Squid

Defender
Landfall — Whenever a land you control enters, this creature can attack this turn as though it didn't have defender.