Marjhan

Marjhan doesn't untap during your untap step.
{U}{U}, Sacrifice a creature: Untap Marjhan. Activate only during your upkeep.
Marjhan can't attack unless defending player controls an Island.
{U}{U}: Marjhan gets -1/-0 until end of turn and deals 1 damage to target attacking creature without flying.
When you control no Islands, sacrifice Marjhan.

Sea Troll

{U}: Regenerate Sea Troll. Activate only if Sea Troll blocked or was blocked by a blue creature this turn.

School of Piranha

At the beginning of your upkeep, sacrifice School of Piranha unless you pay {1}{U}.

Sea Spirit

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

Benthic Behemoth

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

Kukemssa Serpent

Kukemssa Serpent can't attack unless defending player controls an Island.
{U}, Sacrifice an Island: Target land an opponent controls becomes an Island until end of turn.
When you control no Islands, sacrifice Kukemssa Serpent.

Fishliver Oil

Enchant creature (Target a creature as you cast this. This card enters the battlefield attached to that creature.)
Enchanted creature has islandwalk. (It can't be blocked as long as defending player controls an Island.)

Tidal Warrior

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

Segovian Leviathan

Phantasmal Terrain

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

Shrouded Serpent

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

Lingering Mirage

Enchant land
Enchanted land is an Island.
Cycling {2} ({2}, Discard this card: Draw a card.)

Siren's Call

Cast this spell only during an opponent's turn, before attackers are declared.
Creatures the active player controls attack this turn if able.
At the beginning of the next end step, destroy all non-Wall creatures that player controls that didn't attack this turn. Ignore this effect for each creature the player didn't control continuously since the beginning of the turn.

Floodgate

Defender (This creature can't attack.)
When Floodgate has flying, sacrifice it.
When Floodgate leaves the battlefield, it deals damage equal to half the number of Islands you control, rounded down, to each nonblue creature without flying.

Wall of Water

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

Devouring Deep

Tidal Kraken

Sandbar Serpent

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

Giant Oyster

You may choose not to untap Giant Oyster during your untap step.
{T}: For as long as Giant Oyster remains tapped, target tapped creature doesn't untap during its controller's untap step, and at the beginning of each of your draw steps, put a -1/-1 counter on that creature. When Giant Oyster leaves the battlefield or becomes untapped, remove all -1/-1 counters from the creature.

Benthic Explorers

{T}, Untap a tapped land an opponent controls: Add one mana of any type that land could produce.

Giant Albatross

Flying
When Giant Albatross dies, you may pay {1}{U}. If you do, for each creature that dealt damage to Giant Albatross this turn, destroy that creature unless its controller pays 2 life. A creature destroyed this way can't be regenerated.

High Tide

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

Sunken City

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

Hammerhead Shark

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

Dandân

Dandân can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Dandân.

Tangle Kelp

Enchant creature
When Tangle Kelp enters the battlefield, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step if it attacked during its controller's last turn.

Vodalian Knights

First strike
Vodalian Knights can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Vodalian Knights.
{U}: Vodalian Knights gains flying until end of turn.

Coral Atoll

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

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.)

Manta Riders

{U}: Manta Riders gains flying until end of turn.

Man-o'-War

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

Rootwater Commando

Sea Scryer

Veiled Serpent

When an opponent casts a spell, if Veiled Serpent is an enchantment, Veiled Serpent becomes a 4/4 Serpent creature with "This creature can't attack unless defending player controls an Island."
Cycling {2} ({2}, Discard this card: Draw a card.)

Sea Monster

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

Leviathan

Trample
Leviathan enters the battlefield tapped and doesn't untap during your untap step.
At the beginning of your upkeep, you may sacrifice two Islands. If you do, untap Leviathan.
Leviathan can't attack unless you sacrifice two Islands. (This cost is paid as attackers are declared.)

Blue Mana Battery

{2}, {T}: Put a charge counter on Blue Mana Battery.
{T}, Remove any number of charge counters from Blue Mana Battery: Add {U}, then add an additional {U} for each charge counter removed this way.

Killer Whale

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

Flooded Shoreline

{U}{U}, Return two Islands you control to their owner's hand: Return target creature to its owner's hand.

Illusionary Terrain

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.)
As Illusionary Terrain enters the battlefield, choose two basic land types.
Basic lands of the first chosen type are the second chosen type.

Rootwater Hunter

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

Seasinger

When you control no Islands, sacrifice Seasinger.
You may choose not to untap Seasinger during your untap step.
{T}: Gain control of target creature whose controller controls an Island for as long as you control Seasinger and Seasinger remains tapped.

Island Fish Jasconius

Island Fish Jasconius doesn't untap during your untap step.
At the beginning of your upkeep, you may pay {U}{U}{U}. If you do, untap Island Fish Jasconius.
Island Fish Jasconius can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Island Fish Jasconius.

Sea Serpent

Sea Serpent can't attack unless defending player controls an Island.
When you control no Islands, sacrifice Sea Serpent.

High Seas

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

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.

Coral Helm

{3}, Discard a card at random: Target creature gets +2/+2 until end of turn.

Iceberg

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

Merfolk Assassin

{T}: Destroy target creature with islandwalk.

Giant Shark

Giant Shark can't attack unless defending player controls an Island.
Whenever Giant Shark blocks or becomes blocked by a creature that has been dealt damage this turn, Giant Shark gets +2/+0 and gains trample until end of turn.
When you control no Islands, sacrifice Giant Shark.