Rootwater Mystic

{1}{U}: Look at the top card of target player's library.

Annul

Counter target artifact or enchantment spell.

Arctic Merfolk

Kicker—Return a creature you control to its owner's hand. (You may return a creature you control to its owner's hand in addition to any other costs as you cast this spell.)
If Arctic Merfolk was kicked, it enters the battlefield with a +1/+1 counter on it.

Shimmer of Possibility

Look at the top four cards of your library. Put one of them into your hand and the rest on the bottom of your library in a random order.

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.

Sea's Claim

Enchant land (Target a land as you cast this. This card enters the battlefield attached to that land.)
Enchanted land is an Island.

Deeptread Merrow

{U}: Deeptread Merrow gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)

Flood of Recollection

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

Thassa's Devourer

Constellation — Whenever Thassa's Devourer or another enchantment enters the battlefield under your control, target player mills two cards.

Skyline Cascade

Skyline Cascade enters the battlefield tapped.
When Skyline Cascade enters the battlefield, target creature an opponent controls doesn't untap during its controller's next untap step.
{T}: Add {U}.

Disrupt

Counter target instant or sorcery spell unless its controller pays {1}.
Draw a card.

Bonds of Quicksilver

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature doesn't untap during its controller's untap step.

Distant Melody

Choose a creature type. Draw a card for each permanent you control of that type.

Aura Finesse

Attach target Aura you control to target creature.
Draw a card.

Startling Development

Until end of turn, target creature becomes a blue Serpent with base power and toughness 4/4.
Cycling {1} ({1}, Discard this card: Draw a card.)

Arcane Denial

Counter target spell. Its controller may draw up to two cards at the beginning of the next turn's upkeep.
You draw a card at the beginning of the next turn's upkeep.

Merfolk Observer

When Merfolk Observer enters the battlefield, look at the top card of target player's library.

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.

Umara Entangler

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Witching Well

When Witching Well enters the battlefield, scry 2. (Look at the top two cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.)
{3}{U}, Sacrifice Witching Well: Draw two cards.

Island

Tideshaper Mystic

{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.

Opal Palace

{T}: Add {C}.
{1}, {T}: Add one mana of any color in your commander's color identity. If you spend this mana to cast your commander, it enters the battlefield with a number of additional +1/+1 counters on it equal to the number of times it's been cast from the command zone this game.

Courier's Capsule

{1}{U}, {T}, Sacrifice Courier's Capsule: Draw two cards.

Serum Visions

Alchemist's Retrieval

Cleave {1}{U} (You may cast this spell for its cleave cost. If you do, remove the words in square brackets.)
Return target nonland permanent [you control] to its owner's hand.

Scroll Thief

Whenever Scroll Thief deals combat damage to a player, draw a card.

Phantasmal Terrain

Enchant land
As Phantasmal Terrain enters the battlefield, choose a basic land type.
Enchanted land is the chosen type.

Ghostform

Up to two target creatures can't be blocked this turn.

Divination

Counsel of the Soratami

Coral Commando

Inertia Bubble

Enchant artifact
Enchanted artifact doesn't untap during its controller's untap step.

Vodalian Arcanist

{T}: Add {C}. Spend this mana only to cast an instant or sorcery spell.

Dream Thrush

Flying
{T}: Target land becomes the basic land type of your choice until end of turn.

Eel Umbra

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

Mantle of Tides

Equipped creature gets +1/+2.
Whenever you draw your second card each turn, attach Mantle of Tides to target creature you control.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Flight of Fancy

Enchant creature
When Flight of Fancy enters the battlefield, draw two cards.
Enchanted creature has flying.

Counterspell

Negate

Merfolk Spy

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Merfolk Spy deals combat damage to a player, that player reveals a card at random from their hand.

Inkfathom Divers

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Inkfathom Divers enters the battlefield, look at the top four cards of your library, then put them back in any order.

Convincing Mirage

Enchant land
As Convincing Mirage enters the battlefield, choose a basic land type.
Enchanted land is the chosen type.

Bubble Snare

Kicker {2}{U} (You may pay an additional {2}{U} as you cast this spell.)
Enchant creature
When Bubble Snare enters the battlefield, if it was kicked, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.

Reef Shaman

{T}: Target land becomes the basic land type of your choice until end of turn.

Coral Merfolk

Brainstorm

Draw three cards, then put two cards from your hand on top of your library in any order.

Terramorphic Expanse

{T}, Sacrifice Terramorphic Expanse: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Rootwater Hunter

{T}: Rootwater Hunter deals 1 damage to any target.

Soaring Seacliff

Soaring Seacliff enters the battlefield tapped.
When Soaring Seacliff enters the battlefield, target creature gains flying until end of turn.
{T}: Add {U}.

Harbor Serpent

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Harbor Serpent can't attack unless there are five or more Islands on the battlefield.

Rootwater Commando

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)

Spreading Seas

Enchant land
When Spreading Seas enters the battlefield, draw a card.
Enchanted land is an Island.

Path of Ancestry

Path of Ancestry enters the battlefield tapped.
{T}: Add one mana of any color in your commander's color identity. When that mana is spent to cast a creature spell that shares a creature type with your commander, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Mordenkainen's Polymorph

Until end of turn, target creature becomes a Dragon with base power and toughness 4/4 and gains flying. (It loses all other creature types.)

Crystal Grotto

When Crystal Grotto enters the battlefield, scry 1.
{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Merrow Harbinger

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Merrow Harbinger enters the battlefield, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.

Streambed Aquitects

{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.

Arcane Signet

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

Armguard Familiar

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
Equipped creature gets +2/+1 and has ward {2}.
Reconfigure {4} ({4}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Command Tower

Tidal Warrior

{T}: Target land becomes an Island until end of turn.

Mystic Sanctuary

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

Halimar Depths

Halimar Depths enters the battlefield tapped.
When Halimar Depths enters the battlefield, look at the top three cards of your library, then put them back in any order.
{T}: Add {U}.

Void Snare

Return target nonland permanent to its owner's hand.

Salvager of Secrets

When Salvager of Secrets enters the battlefield, return target instant or sorcery card from your graveyard to your hand.

Remote Isle

Remote Isle enters the battlefield tapped.
{T}: Add {U}.
Cycling {2} ({2}, Discard this card: Draw a card.)

Otherworldly Gaze

Surveil 3. (Look at the top three cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)
Flashback {1}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Augur of Bolas

When Augur of Bolas enters the battlefield, look at the top three cards of your library. You may reveal an instant or sorcery card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Unsummon

Return target creature to its owner's hand.

Preordain

Scry 2, then draw a card. (To scry 2, look at the top two cards of your library, then put any number of them on the bottom and the rest on top in any order.)