Cut Your Losses

Casualty 2 (As you cast this spell, you may sacrifice a creature with power 2 or greater. When you do, copy this spell and you may choose a new target for the copy.)
Target player mills half their library, rounded down.

Have

Favor of Jukai

Enchant artifact or creature
As long as enchanted permanent is a creature, it gets +3/+3 and has reach.
Channel — {1}{G}, Discard Favor of Jukai: Target creature gets +3/+3 and gains reach until end of turn.

True-Name Nemesis

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

Sea God's Revenge

Return up to three target creatures your opponents control to their owners' hands. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

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.

Lord of Atlantis

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

Woodland Stream

Woodland Stream enters tapped.
{T}: Add {G} or {U}.

Flood of Recollection

Return target instant or sorcery card from your graveyard to your hand. Exile Flood of Recollection.

Wurm's Tooth

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

Kiora's Follower

Stern Dismissal

Return target creature or enchantment an opponent controls to its owner's hand.

Harbinger of the Tides

You may cast this card 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 this creature enters, you may return target tapped creature an opponent controls to its owner's hand.

Crashing Tide

This spell has flash as long as you control a Merfolk.
Return target creature to its owner's hand.
Draw a card.

Steady Progress

Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Draw a card.

Gaea's Skyfolk

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.)

Thornwood Falls

This land enters tapped.
When this land enters, you gain 1 life.
{T}: Add {G} or {U}.

Forest

Merfolk Mistbinder

Basilisk Collar

Equipped creature has deathtouch and lifelink. (Any amount of damage it deals to a creature is enough to destroy it. Damage dealt by this creature also causes you to gain that much life.)
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Merrow Commerce

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

Sigiled Starfish

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

Thassa's Oracle

When Thassa's Oracle enters, look at the top X cards of your library, where X is your devotion to blue. Put up to one of them on top of your library and the rest on the bottom of your library in a random order. If X is greater than or equal to the number of cards in your library, you win the game. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

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.)

Lay of the Land

Search your library for a basic land card, reveal it, put it into your hand, then shuffle.

Island

Moonlit Scavengers

When Moonlit Scavengers enters, if you control an artifact or enchantment, return target creature an opponent controls to its owner's hand.

Simic Signet

Cartouche of Knowledge

Enchant creature you control
When Cartouche of Knowledge enters, draw a card.
Enchanted creature gets +1/+1 and has flying.

Towering-Wave Mystic

Whenever Towering-Wave Mystic deals damage, target player mills that many cards.

Emergent Growth

Target creature gets +5/+5 until end of turn and must be blocked this turn if able.

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.)

Counterspell

Hullbreacher

Flash
If an opponent would draw a card except the first one they draw in each of their draw steps, instead you create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Riverwise Augur

When Riverwise Augur enters, draw three cards, then put two cards from your hand on top of your library in any order.

World Shaper

Whenever World Shaper attacks, you may mill three cards.
When World Shaper dies, return all land cards from your graveyard to the battlefield tapped.

Swift Warden

Flash
When Swift Warden enters, target Merfolk you control gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Wishful Merfolk

Defender
{1}{U}: Wishful Merfolk loses defender and becomes a Human until end of turn.

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.)

Merrow Reejerey

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

Cartouche of Strength

Enchant creature you control
When Cartouche of Strength enters, you may have enchanted creature fight target creature an opponent controls. (Each deals damage equal to its power to the other.)
Enchanted creature gets +1/+1 and has trample.

Tatyova, Benthic Druid

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

Talrand, Sky Summoner

Whenever you cast an instant or sorcery spell, create a 2/2 blue Drake creature token with flying.

Vexing Scuttler

Emerge {6}{U} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When you cast this spell, you may return target instant or sorcery card from your graveyard to your hand.

Rampaging Hippo

Trample
Cycling {2} ({2}, Discard this card: Draw a card.)

Herald of Secret Streams

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

Deeproot Elite

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

Compelled Duel

Target creature gets +3/+3 until end of turn and must be blocked this turn if able.

Shed Weakness

Target creature gets +2/+2 until end of turn. You may remove a -1/-1 counter from it.