Castle Vantress

Castle Vantress enters tapped unless you control an Island.
{T}: Add {U}.
{2}{U}{U}, {T}: Scry 2.

Aboleth Spawn

Flash
Ward {2}
Probing Telepathy — Whenever a creature entering under an opponent's control causes a triggered ability of that creature to trigger, you may copy that ability. You may choose new targets for the copy.

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.

Piranha Fly

Man-o'-War

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

Stinging Lionfish

Whenever you cast your first spell during each opponent's turn, you may tap or untap target nonland permanent.

Skyswimmer Koi

Flying
Whenever an artifact you control enters, you may draw a card. If you do, discard a card.

Island

Windrider Eel

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

Tidal Terror

Whenever Tidal Terror attacks, you may tap two other untapped creatures you control. If you do, Tidal Terror can't be blocked this turn.
Islandcycling {2} ({2}, Discard this card: Search your library for an Island card, reveal it, put it into your hand, then shuffle.)

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.

Cephalid Coliseum

{T}: Add {U}. Cephalid Coliseum deals 1 damage to you.
Threshold — {U}, {T}, Sacrifice Cephalid Coliseum: Target player draws three cards, then discards three cards. Activate only if seven or more cards are in your graveyard.

Coral Eel

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.)

Mystic Skyfish

Whenever you draw your second card each turn, Mystic Skyfish gains flying until end of turn.

Skitter Eel

{2}{U}: Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)

Deepglow Skate

When Deepglow Skate enters, double the number of each kind of counter on any number of target permanents.

Guard Gomazoa

Defender, flying
Prevent all combat damage that would be dealt to Guard Gomazoa.

Breaching Leviathan

When Breaching Leviathan enters, if you cast it from your hand, tap all nonblue creatures. Those creatures don't untap during their controllers' next untap steps.

Jace's Mindseeker

Flying
When Jace's Mindseeker enters, target opponent mills five cards. You may cast an instant or sorcery spell from among them without paying its mana cost.

Wavebreak Hippocamp

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

Vivid Flying Fish

Vivid Flying Fish has flying as long as it's attacking. (It can't be blocked except by creatures with flying or reach.)

Inkwell Leviathan

Trample
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Shroud (This creature can't be the target of spells or abilities.)

Shipwreck Moray

When Shipwreck Moray enters, you get {E}{E}{E}{E} (four energy counters).
Pay {E}: Shipwreck Moray gets +2/-2 until end of turn.

Star Whale

Flying, vigilance
Other creatures you control have ward {2}.
Suspend 6—{1}{U} (Rather than cast this card from your hand, you may pay {1}{U} and exile it with six time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost. It has haste.)

Sky-Eel School

Flying
When Sky-Eel School enters, draw a card, then discard a card.

Moat Piranhas

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.

Slipstream Eel

Slipstream Eel can't attack unless defending player controls an Island.
Cycling {1}{U} ({1}{U}, Discard this card: Draw a card.)

Kaito, Cunning Infiltrator

Whenever a creature you control deals combat damage to a player, put a loyalty counter on Kaito.
+1: Up to one target creature you control can't be blocked this turn. Draw a card, then discard a card.
−2: Create a 2/1 blue Ninja creature token.
−9: You get an emblem with "Whenever a player casts a spell, you create a 2/1 blue Ninja creature token."

Ancient Carp

Sleep with the Fishes

Enchant creature
When Sleep with the Fishes enters, tap enchanted creature and you create a 1/1 blue Fish creature token with "This creature can't be blocked."
Enchanted creature doesn't untap during its controller's untap step.

Fleet Swallower

Whenever Fleet Swallower attacks, target player mills half their library, rounded up.

Cloud Manta

Jace, Unraveler of Secrets

+1: Scry 1, then draw a card.
−2: Return target creature to its owner's hand.
−8: You get an emblem with "Whenever an opponent casts their first spell each turn, counter that spell."

Kapsho Kitefins

Flying
Whenever Kapsho Kitefins or another creature you control enters, tap target creature an opponent controls.

Reservoir Kraken

Trample, ward {2}
At the beginning of each combat, if Reservoir Kraken is untapped, any opponent may tap an untapped creature they control. If they do, tap Reservoir Kraken and create a 1/1 blue Fish creature token with "This creature can't be blocked."

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.'"

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.

Bubble Smuggler

Disguise {5}{U} (You may cast this card face down for {3} as a 2/2 creature with ward {2}. Turn it face up any time for its disguise cost.)
As Bubble Smuggler is turned face up, put four +1/+1 counters on it.

Tidal Barracuda

Any player may cast spells as though they had flash.
Your opponents can't cast spells during your turn.

Expendable Lackey

{1}{U}, Exile Expendable Lackey from your graveyard: Create a 1/1 blue Fish creature token with "This creature can't be blocked." Activate only as a sorcery.

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.