Have

Kiora's Dambreaker

When Kiora's Dambreaker enters, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Phase Dolphin

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

Wishful Merfolk

Defender
{1}{U}: Wishful Merfolk loses defender and becomes a Human until end of turn.

Negate

Horseshoe Crab

Dispel

Essence Scatter

Stealth Mission

Put two +1/+1 counters on target creature you control. That creature can't be blocked this turn.

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.)

Crush Dissent

Counter target spell unless its controller pays {2}.
Amass Zombies 2. (Put two +1/+1 counters on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)

Capsizing Wave

Return target creature to its owner's hand. (Then exile this card. You may cast the creature later from exile.)

Sword Coast Serpent

Sword Coast Serpent can't be blocked as long as you've cast a noncreature spell this turn.

Sword Coast Serpent // Capsizing Wave

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.)

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.

Island

Didn't Say Please

Counter target spell. Its controller mills three cards.

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.)

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.)

Convolute

Counter target spell unless its controller pays {4}.

Thriving Turtle

When Thriving Turtle enters, you get {E}{E} (two energy counters).
Whenever Thriving Turtle attacks, you may pay {E}{E}. If you do, put a +1/+1 counter on it.

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.

Tidewater Minion

Defender (This creature can't attack.)
{4}: Tidewater Minion loses defender until end of turn.
{T}: Untap target permanent.

Mistford River Turtle

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

Gulf Squid

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

Windrider Eel

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

Skyclave Squid

Defender
Landfall — Whenever a land you control enters, Skyclave Squid can attack this turn as though it didn't have defender.

Marauding Brinefang

Ward {3} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {3}.)
Islandcycling {2} ({2}, Discard this card: Search your library for an Island card, reveal it, put it into your hand, then shuffle.)

Grayscaled Gharial

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

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.

Tidal Visionary

{T}: Target creature becomes the color of your choice until end of turn.

Naiad of Hidden Coves

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

Cancel

Moat Piranhas

Aegis Turtle

Crystacean

Flash (You may cast this spell any time you could cast an instant.)

Hammerhead Shark

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

Hightide Hermit

Defender
When Hightide Hermit enters, you get {E}{E}{E}{E} (four energy counters).
Pay {E}{E}: Hightide Hermit can attack this turn as though it didn't have defender.