Narwhal

Hammerhead Shark

Hammerhead Shark can't attack unless defending player controls an Island.

Gulf Squid

When Gulf Squid enters, tap all lands target player controls.

Ice Floe

You may choose not to untap Ice Floe during your untap step.
{T}: Tap target creature without flying that's attacking you. It doesn't untap during its controller's untap step for as long as Ice Floe remains tapped.

Dreamwinder

Dreamwinder can't attack unless defending player controls an Island.
{U}, Sacrifice an Island: Target land becomes an Island until end of turn.

Island

Living Tsunami

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

Thassa's Ire

{3}{U}: You may tap or untap target creature.

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.

Ponder

Look at the top three cards of your library, then put them back in any order. You may shuffle.
Draw a card.

Tidal Surge

Tap up to three target creatures without flying.

Faerie Conclave

Faerie Conclave enters tapped.
{T}: Add {U}.
{1}{U}: Faerie Conclave becomes a 2/1 blue Faerie creature with flying until end of turn. It's still a land.

Thassa, God of the Sea

Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
At the beginning of your upkeep, scry 1.
{1}{U}: Target creature you control can't be blocked this turn.

Sea Spirit

{U}: Sea Spirit gets +1/+0 until end of turn.

Thassa's Emissary

Bestow {5}{U} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Whenever Thassa's Emissary or enchanted creature deals combat damage to a player, draw a card.
Enchanted creature gets +3/+3.

King Crab

{1}{U}, {T}: Put target green creature on top of its owner's library.

Tromokratis

Tromokratis has hexproof unless it's attacking or blocking.
Tromokratis can't be blocked unless all creatures defending player controls block it. (If any creature that player controls doesn't block this creature, it can't be blocked.)

Whitewater Naiads

Constellation — Whenever Whitewater Naiads or another enchantment you control enters, target creature can't be blocked this turn.

Breaking Wave

You may cast Breaking Wave as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
Simultaneously untap all tapped creatures and tap all untapped creatures.

High Seas

Red creature spells and green creature spells cost {1} more to cast.

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.

Iceberg

Iceberg enters with X ice counters on it.
{3}: Put an ice counter on Iceberg.
Remove an ice counter from Iceberg: Add {C}.

Wall of Kelp

Defender (This creature can't attack.)
{U}{U}, {T}: Create a 0/1 blue Plant Wall creature token with defender named Kelp.

Displacement Wave

Return all nonland permanents with mana value X or less to their owners' hands.

Moonring Island

({T}: Add {U}.)
Moonring Island enters tapped.
{U}, {T}: Look at the top card of target player's library. Activate only if you control two or more blue permanents.

Wall of Water

Defender (This creature can't attack.)
{U}: Wall of Water gets +1/+0 until end of turn.

Whelming Wave

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

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.

Sea Sprite

Devouring Deep

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

Benthic Behemoth

Pirate Ship

Pirate Ship can't attack unless defending player controls an Island.
{T}: Pirate Ship deals 1 damage to any target.
When you control no Islands, sacrifice Pirate Ship.

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.)

Colossal Whale

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Colossal Whale attacks, you may exile target creature defending player controls until Colossal Whale leaves the battlefield. (That creature returns under its owner's control.)

Sunken City

At the beginning of your upkeep, sacrifice Sunken City unless you pay {U}{U}.
Blue creatures get +1/+1.

Sigiled Starfish

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

Merfolk Assassin

{T}: Destroy target creature with islandwalk.

Crystalline Nautilus

Bestow {3}{U}{U} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
When Crystalline Nautilus becomes the target of a spell or ability, sacrifice it.
Enchanted creature gets +4/+4 and has "When this creature becomes the target of a spell or ability, sacrifice it."

Ordeal of Thassa

Enchant creature
Whenever enchanted creature attacks, put a +1/+1 counter on it. Then if it has three or more +1/+1 counters on it, sacrifice Ordeal of Thassa.
When you sacrifice Ordeal of Thassa, draw two cards.

Reliquary Tower

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

Sea Snidd

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

Thassa's Rebuff

Counter target spell unless its controller pays {X}, where X is your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Coastal Piracy

Whenever a creature you control deals combat damage to an opponent, you may draw a card.

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}.

Thassa's Bounty

Draw three cards. Target player mills three cards.

Soaring Seacliff

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

Tidal Control

Cumulative upkeep {2} (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.)
Pay 2 life or {2}: Counter target red or green spell. Any player may activate this ability.

Inundate

Return all nonblue creatures to their owners' hands.

Thassa's Devourer

Constellation — Whenever Thassa's Devourer or another enchantment you control enters, target player mills two cards.

Killer Whale

{U}: Killer Whale gains flying until end of turn.

Flow of Ideas

Part Water

X target creatures gain islandwalk until end of turn. (They can't be blocked as long as defending player controls an Island.)

Crush of Tentacles

Surge {3}{U}{U} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Return all nonland permanents to their owners' hands. If this spell's surge cost was paid, create an 8/8 blue Octopus creature token.

War Barge

{3}: Target creature gains islandwalk until end of turn. When War Barge leaves the battlefield this turn, destroy that creature. A creature destroyed this way can't be regenerated. (A creature with islandwalk can't be blocked as long as defending player controls an Island.)

Segovian Leviathan

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.

Sliptide Serpent

{3}{U}: Return Sliptide Serpent to its owner's hand.

Svyelunite Temple

Svyelunite Temple enters tapped.
{T}: Add {U}.
{T}, Sacrifice Svyelunite Temple: Add {U}{U}.

Shrouded Serpent

Whenever Shrouded Serpent attacks, defending player may pay {4}. If that player doesn't, Shrouded Serpent can't be blocked this turn.

Sea Monster

Sea Monster can't attack unless defending player controls an Island.

Submerge

If an opponent controls a Forest and you control an Island, you may cast this spell without paying its mana cost.
Put target creature on top of its owner's library.

Sealock Monster

Sealock Monster can't attack unless defending player controls an Island.
{5}{U}{U}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When Sealock Monster becomes monstrous, target land becomes an Island in addition to its other types.

Deep-Sea Terror

Deep-Sea Terror can't attack unless there are seven or more cards in your graveyard.

Coral Barrier

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

Spiny Starfish

{U}: Regenerate Spiny Starfish.
At the beginning of each end step, if Spiny Starfish regenerated this turn, create a 0/1 blue Starfish creature token for each time it regenerated this turn.

Temple of the False God

{T}: Add {C}{C}. Activate only if you control five or more lands.

Denizen of the Deep

When Denizen of the Deep enters, return each other creature you control to its owner's hand.

Manta Ray

Manta Ray can't attack unless defending player controls an Island.
Manta Ray can't be blocked except by blue creatures.
When you control no Islands, sacrifice Manta Ray.

Kraken of the Straits

Creatures with power less than the number of Islands you control can't block Kraken of the Straits.

High Tide

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