Have

Stormtide Leviathan

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
All lands are Islands in addition to their other types.
Creatures without flying or islandwalk can't attack.

Frilled Cave-Wurm

Descend 4 — This creature gets +2/+0 as long as there are four or more permanent cards in your graveyard.

Dream Fracture

Counter target spell. Its controller draws a card.
Draw a card.

Refute

Counter target spell. Draw a card, then discard a card.

Kumena, Tyrant of Orazca

Tap another untapped Merfolk you control: Kumena 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.

Simic Ascendancy

{1}{G}{U}: Put a +1/+1 counter on target creature you control.
Whenever one or more +1/+1 counters are put on a creature you control, put that many growth counters on this enchantment.
At the beginning of your upkeep, if this enchantment has twenty or more growth counters on it, you win the game.

Flow of Knowledge

Draw a card for each Island you control, then discard two cards.

Bender's Waterskin

Untap this artifact during each other player's untap step.
{T}: Add one mana of any color.

Kiora, Master of the Depths

+1: Untap up to one target creature and up to one target land.
−2: Reveal the top four cards of your library. You may put a creature card and/or a land card from among them into your hand. Put the rest into your graveyard.
−8: You get an emblem with "Whenever a creature you control enters, you may have it fight target creature." Then create three 8/8 blue Octopus creature tokens.

Vodalian Hexcatcher

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

Swarm Intelligence

Whenever you cast an instant or sorcery spell, you may copy that spell. You may choose new targets for the copy.

Koma, World-Eater

This spell can't be countered.
Trample, ward {4}
Whenever Koma deals combat damage to a player, create four 3/3 blue Serpent creature tokens named Koma's Coil.

Quicksilver Fountain

At the beginning of each player's upkeep, that player puts a flood counter on target non-Island land they control of their choice. That land is an Island for as long as it has a flood counter on it.
At the beginning of each end step, if all lands on the battlefield are Islands, remove all flood counters from them.

Kiora, the Rising Tide

When Kiora enters, draw two cards, then discard two cards.
Threshold — Whenever Kiora attacks, if there are seven or more cards in your graveyard, you may create Scion of the Deep, a legendary 8/8 blue Octopus creature token.

Vivid Grove

This land enters tapped with two charge counters on it.
{T}: Add {G}.
{T}, Remove a charge counter from this land: Add one mana of any color.

Command Tower

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

Seafloor Oracle

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

Sol Ring

Merfolk Sovereign

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

Xolatoyac, the Smiling Flood

Whenever Xolatoyac enters or attacks, 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.
At the beginning of your end step, untap each permanent you control with a counter on it.

Frantic Search

Draw two cards, then discard two cards. Untap up to three lands.

Dig Through Time

Delve (Each card you exile from your graveyard while casting this spell pays for {1}.)
Look at the top seven cards of your library. Put two of them into your hand and the rest on the bottom of your library in any order.

Reliquary Tower

You have no maximum hand size.
{T}: Add {C}.

Hakbal of the Surging Soul

At the beginning of combat on your turn, each Merfolk creature you control 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 the exploring creature, then put the card back or put it into your graveyard.)
Whenever Hakbal attacks, you may put a land card from your hand onto the battlefield. If you don't, draw a card.

Tatyova, Benthic Druid

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

Don't Have

Mana Drain

Counter target spell. At the beginning of your next main phase, add an amount of {C} equal to that spell's mana value.

Tangled Islet

({T}: Add {G} or {U}.)
This land enters tapped.

Thought Vessel

Koma, Cosmos Serpent

This spell can't be countered.
At the beginning of each upkeep, create a 3/3 blue Serpent creature token named Koma's Coil.
Sacrifice another Serpent: Choose one —
• Tap target permanent. Its activated abilities can't be activated this turn.
• Koma gains indestructible until end of turn.

Merrow Reejerey

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

Explore

You may play an additional land this turn.
Draw a card.

Forest

Evacuation

Return all creatures to their owners' hands.

Island

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Eluge, the Shoreless Sea

Eluge's power and toughness are each equal to the number of Islands you control.
Whenever Eluge enters or attacks, put a flood counter on target land. It's an Island in addition to its other types for as long as it has a flood counter on it.
The first instant or sorcery spell you cast each turn costs {U} (or {1}) less to cast for each land you control with a flood counter on it.

Auroral Procession

Return target card from your graveyard to your hand.

Pull from the Deep

Return up to one target instant card and up to one target sorcery card from your graveyard to your hand. Exile Pull from the Deep.

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.)

Waterbending Scroll

{6}, {T}: Draw a card. This ability costs {1} less to activate for each Island you control.

Nesting Grounds

{T}: Add {C}.
{1}, {T}: Move a counter from target permanent you control onto a second target permanent. Activate only as a sorcery.

Cold-Eyed Selkie

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever this creature deals combat damage to a player, you may draw that many cards.

Long River's Pull

Gift a card (You may promise an opponent a gift as you cast this spell. If you do, they draw a card before its other effects.)
Counter target creature spell. If the gift was promised, instead counter target spell.

Aetherspouts

For each attacking creature, its owner puts it on their choice of the top or bottom of their library.

Vivid Creek

This land enters tapped with two charge counters on it.
{T}: Add {U}.
{T}, Remove a charge counter from this land: Add one mana of any color.

Fleeting Distraction

Target creature gets -1/-0 until end of turn.
Draw a card.

Simic Growth Chamber

This land enters tapped.
When this land enters, return a land you control to its owner's hand.
{T}: Add {G}{U}.

Think Twice

Draw a card.
Flashback {2}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Mist Dancer

Flying
Other Merfolk you control get +1/+0 and have flying.
Encore {5}{U}{U} ({5}{U}{U}, Exile this card from your graveyard: For each opponent, create a token copy that attacks that opponent this turn if able. They gain haste. Sacrifice them at the beginning of the next end step. Activate only as a sorcery.)

Thriving Isle

This land enters tapped. As it enters, choose a color other than blue.
{T}: Add {U} or one mana of the chosen color.

Evolution Sage

Landfall — Whenever a land you control enters, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Bounty of the Luxa

At the beginning of your first main phase, remove all flood counters from this enchantment. If no counters were removed this way, put a flood counter on this enchantment and draw a card. Otherwise, add {C}{G}{U}.

Arixmethes, Slumbering Isle

Arixmethes enters tapped with five slumber counters on it.
As long as Arixmethes has a slumber counter on it, it's a land. (It's not a creature.)
Whenever you cast a spell, you may remove a slumber counter from Arixmethes.
{T}: Add {G}{U}.

Vorel of the Hull Clade

{G}{U}, {T}: Double the number of each kind of counter on target artifact, creature, or land.

Nicanzil, Current Conductor

Whenever a creature you control explores a land card, you may put a land card from your hand onto the battlefield tapped.
Whenever a creature you control explores a nonland card, put a +1/+1 counter on Nicanzil.

Rain of Revelation

Exotic Orchard

{T}: Add one mana of any color that a land an opponent controls could produce.

Flood of Tears

Return all nonland permanents to their owners' hands. If you return four or more nontoken permanents you control this way, you may put a permanent card from your hand onto the battlefield.

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.)

Thornwood Falls

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

Prevent all combat damage that would be dealt this turn.

Witness Protection

Enchant creature
Enchanted creature loses all abilities and is a green and white Citizen creature with base power and toughness 1/1 named Legitimate Businessperson. (It loses all other colors, card types, creature types, and names.)

Deeproot Elite

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

Master Biomancer

Each other creature you control enters with a number of additional +1/+1 counters on it equal to this creature's power and as a Mutant in addition to its other types.

Wash Out

Return all permanents of the color of your choice to their owners' hands.

Bigfin Bouncer

When this creature enters, return target creature an opponent controls to its owner's hand.

Inspiring Call

Draw a card for each creature you control with a +1/+1 counter on it. Those creatures gain indestructible until end of turn. (Damage and effects that say "destroy" don't destroy them.)

Engulf the Shore

Return to their owners' hands all creatures with toughness less than or equal to the number of Islands you control.

Hinterland Harbor

This land enters tapped unless you control a Forest or an Island.
{T}: Add {G} or {U}.

Calamitous Tide

Return up to two target creatures to their owners' hands. Draw two cards, then discard a card.

Dark Depths

Dark Depths enters with ten ice counters on it.
{3}: Remove an ice counter from Dark Depths.
When Dark Depths has no ice counters on it, sacrifice it. If you do, create Marit Lage, a legendary 20/20 black Avatar creature token with flying and indestructible.

Gale, Waterdeep Prodigy

Whenever you cast an instant or sorcery spell from your hand, you may cast up to one target card of the other type from your graveyard. If a spell cast from your graveyard this way would be put into your graveyard, exile it instead.
Choose a Background (You can have a Background as a second commander.)

Negate

High Tide

Until end of turn, whenever a player taps an Island for mana, that player adds an additional {U}.

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.

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.

Archmage of Runes

Instant and sorcery spells you cast cost {1} less to cast.
Whenever you cast an instant or sorcery spell, draw a card.

Spreading Seas

Enchant land
When this Aura enters, draw a card.
Enchanted land is an Island.

Mystic Sanctuary

({T}: Add {U}.)
This land enters tapped unless you control three or more other Islands.
When this land enters untapped, you may put target instant or sorcery card from your graveyard on top of your library.

Flooded Grove

{T}: Add {C}.
{G/U}, {T}: Add {G}{G}, {G}{U}, or {U}{U}.

Contentious Plan

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