Ichthyomorphosis

Enchant creature
Enchanted creature loses all abilities and is a blue Fish with base power and toughness 0/1.

Spell Pierce

Counter target noncreature spell unless its controller pays {2}.

Omen of the Sea

Flash
When Omen of the Sea enters, scry 2, then draw a card.
{2}{U}, Sacrifice Omen of the Sea: Scry 2.

Aeromoeba

Flying
Discard a card: Switch Aeromoeba's power and toughness until end of turn.

Burrog Befuddler

Flash
When Burrog Befuddler enters, target creature an opponent controls gets -1/-0 until end of turn.

Apprentice Wizard

Surge Mare

Surge Mare can't be blocked by green creatures.
Whenever Surge Mare deals damage to an opponent, you may draw a card. If you do, discard a card.
{1}{U}: Surge Mare gets +2/-2 until end of turn.

Aerial Formation

Strive — This spell costs {2}{U} more to cast for each target beyond the first.
Any number of target creatures each get +1/+1 and gain flying until end of turn.

Dive Down

Target creature you control gets +0/+3 and gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Reef Worm

When Reef Worm dies, create a 3/3 blue Fish creature token with "When this creature dies, create a 6/6 blue Whale creature token with 'When this creature dies, create a 9/9 blue Kraken creature token.'"

Roost of Drakes

Kicker {2}{U} (You may pay an additional {2}{U} as you cast this spell.)
When Roost of Drakes enters, if it was kicked, create a 2/2 blue Drake creature token with flying.
Whenever you cast a kicked spell, create a 2/2 blue Drake creature token with flying.

Pursued Whale

When Pursued Whale enters, each opponent creates a 1/1 red Pirate creature token with "This creature can't block" and "Creatures you control attack each combat if able."
Spells your opponents cast that target Pursued Whale cost {3} more to cast.

Man-o'-War

When Man-o'-War enters, return target creature to its owner's hand.

Nadir Kraken

Whenever you draw a card, you may pay {1}. If you do, put a +1/+1 counter on Nadir Kraken and create a 1/1 blue Tentacle creature token.

Sigiled Starfish

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

Oneirophage

Flying
Whenever you draw a card, put a +1/+1 counter on Oneirophage.

Interpret the Signs

Scry 3, then reveal the top card of your library. Draw cards equal to that card's mana value. (To scry 3, look at the top three cards of your library, then put any number of them on the bottom and the rest on top in any order.)

Callaphe, Beloved of the Sea

Callaphe's power is equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
Creatures and enchantments you control have "Spells your opponents cast that target this permanent cost {1} more to cast."

Consuming Tide

Each player chooses a nonland permanent they control. Return all nonland permanents not chosen this way to their owners' hands. Then you draw a card for each opponent who has more cards in their hand than you.

Clinging Anemones

Defender
Evolve (Whenever a creature enters under your control, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Island

Wash Away

Cleave {1}{U}{U} (You may cast this spell for its cleave cost. If you do, remove the words in square brackets.)
Counter target spell [that wasn't cast from its owner's hand].

Aqueous Form

Enchant creature
Enchanted creature can't be blocked.
Whenever enchanted creature attacks, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Hall of Storm Giants

If you control two or more other lands, Hall of Storm Giants enters tapped.
{T}: Add {U}.
{5}{U}: Until end of turn, Hall of Storm Giants becomes a 7/7 blue Giant creature with ward {3}. It's still a land. (Whenever it becomes the target of a spell or ability an opponent controls, counter it unless that player pays {3}.)

Chorus of the Tides

Flying
Heroic — Whenever you cast a spell that targets Chorus of the Tides, scry 1. (To scry 1, look at the top card of your library, then you may put that card on the bottom.)

Long-Finned Skywhale

Flying
Long-Finned Skywhale can block only creatures with flying.

Tolarian Kraken

Whenever you draw a card, you may pay {1}. When you do, you may tap or untap target creature.

Ominous Seas

Whenever you draw a card, put a foreshadow counter on Ominous Seas.
Remove eight foreshadow counters from Ominous Seas: Create an 8/8 blue Kraken creature token.
Cycling {2} ({2}, Discard this card: Draw a card.)

Glimmerbell

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.

Mystic Reflection

Choose target nonlegendary creature. The next time one or more creatures or planeswalkers enter this turn, they enter as copies of the chosen creature.
Foretell {U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Voyage's End

Return target creature to its owner's hand. Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Naiad of Hidden Coves

During turns other than yours, spells you cast cost {1} less to cast.

Charix, the Raging Isle

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

Wayfarer's Bauble

{2}, {T}, Sacrifice Wayfarer's Bauble: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Slinn Voda, the Rising Deep

Kicker {1}{U} (You may pay an additional {1}{U} as you cast this spell.)
When Slinn Voda, the Rising Deep enters, if it was kicked, return all creatures to their owners' hands except for Merfolk, Krakens, Leviathans, Octopuses, and Serpents.

Breaching Hippocamp

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

Serpentine Ambush

Until end of turn, target creature becomes a blue Serpent with base power and toughness 5/5.

Windrider Eel

Flying
Landfall — Whenever a land you control enters, Windrider Eel gets +2/+2 until end of turn.

Thieving Otter

Whenever Thieving Otter deals damage to an opponent, draw a card.

Wormhole Serpent

{3}{U}: Target creature can't be blocked this turn.

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.

Spined Megalodon

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Whenever Spined Megalodon attacks, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Thirst for Meaning

Draw three cards. Then discard two cards unless you discard an enchantment card.

Wavebreak Hippocamp

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

Aegis Turtle

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.)

Mistford River Turtle

Whenever Mistford River Turtle attacks, another target attacking non-Human creature can't be blocked this turn.

Silent Submersible

Whenever Silent Submersible deals combat damage to a player or planeswalker, draw a card.
Crew 2 (Tap any number of creatures you control with total power 2 or more: This Vehicle becomes an artifact creature until end of turn.)

Shell Shield

Kicker {1} (You may pay an additional {1} as you cast this spell.)
Target creature you control gets +0/+3 until end of turn. If this spell was kicked, that creature also gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Turtleshell Changeling

Changeling (This card is every creature type.)
{1}{U}: Switch Turtleshell Changeling's power and toughness until end of turn.

Voracious Greatshark

Flash
When Voracious Greatshark enters, counter target artifact or creature spell.

Ior Ruin Expedition

Landfall — Whenever a land you control enters, you may put a quest counter on Ior Ruin Expedition.
Remove three quest counters from Ior Ruin Expedition and sacrifice it: Draw two cards.

Water Servant

{U}: Water Servant gets +1/-1 until end of turn.
{U}: Water Servant gets -1/+1 until end of turn.

Forceful Denial

Cascade (When you cast this spell, exile cards from the top of your library until you exile a nonland card that costs less. You may cast it without paying its mana cost. Put the exiled cards on the bottom in a random order.)
Counter target spell.

Dragon Turtle

Flash
Drag Below — When Dragon Turtle enters, tap it and up to one target creature an opponent controls. They don't untap during their controllers' next untap steps.

Scourge of Fleets

When Scourge of Fleets enters, return each creature your opponents control with toughness X or less to its owner's hand, where X is the number of Islands you control.

Bubble Snare

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

Phase Dolphin

Whenever Phase Dolphin attacks, another target attacking creature can't be blocked this turn.

Cryptic Serpent

This spell costs {1} less to cast for each instant and sorcery card in your graveyard.

Leyline of Anticipation

If Leyline of Anticipation is in your opening hand, you may begin the game with it on the battlefield.
You may cast spells as though they had flash.

Mesmerizing Benthid

When Mesmerizing Benthid enters, create two 0/2 blue Illusion creature tokens with "Whenever this creature blocks a creature, that creature doesn't untap during its controller's next untap step."
Mesmerizing Benthid has hexproof as long as you control an Illusion.

Drain Power

Target player activates a mana ability of each land they control. Then that player loses all unspent mana and you add the mana lost this way.

Spectral Sailor

Flash (You may cast this spell any time you could cast an instant.)
Flying
{3}{U}: Draw a card.

Nascent Metamorph

Whenever Nascent Metamorph attacks or blocks, target opponent reveals cards from the top of their library until they reveal a creature card. Nascent Metamorph becomes a copy of that card until end of turn. Then that player puts all cards revealed this way on the bottom of their library in a random order.

Coral Atoll

Coral Atoll enters tapped.
When Coral Atoll enters, sacrifice it unless you return an untapped Island you control to its owner's hand.
{T}: Add {C}{U}.

Octoprophet

When Octoprophet enters, 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.)

Sol Ring

Sea-Dasher Octopus

Mutate {1}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Whenever this creature deals combat damage to a player, draw a card.

Drawn from Dreams

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 a random order.

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 Brineborn Cutthroat.

Whelming Wave

Return all creatures to their owners' hands except for Krakens, Leviathans, Octopuses, and Serpents.

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.

Halimar Depths

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

Nezahal, Primal Tide

This spell can't be countered.
You have no maximum hand size.
Whenever an opponent casts a noncreature spell, draw a card.
Discard three cards: Exile Nezahal, Primal Tide. Return it to the battlefield tapped under its owner's control at the beginning of the next end step.

Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

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.

Mulldrifter

Flying
When Mulldrifter enters, draw two cards.
Evoke {2}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters.)

Mystic Remora

Cumulative upkeep {1} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Whenever an opponent casts a noncreature spell, you may draw a card unless that player pays {4}.

Lorthos, the Tidemaker

Whenever Lorthos, the Tidemaker attacks, you may pay {8}. If you do, tap up to eight target permanents. Those permanents don't untap during their controllers' next untap steps.

Riptide Turtle

Tide Skimmer

Flying
Whenever you attack with two or more creatures with flying, draw a card.

Staff of the Mind Magus

Whenever you cast a blue spell or an Island you control enters, you gain 1 life.