Breaching Hippocamp

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

Countermand

Counter target spell. Its controller mills four cards.

Sphinx's Disciple

Flying
Inspired — Whenever Sphinx's Disciple becomes untapped, draw a card.

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 of your library.)

Deny the Divine

Counter target creature or enchantment spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Vexing Gull

Ichthyomorphosis

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

Mnemonic Wall

Defender
When Mnemonic Wall enters the battlefield, you may return target instant or sorcery card from your graveyard to your hand.

Hubris

Return target creature and all Auras attached to it to their owners' hands.

Island

Towering-Wave Mystic

Whenever Towering-Wave Mystic deals damage, target player mills that many cards.

Thassa's Bounty

Draw three cards. Target player mills three cards.

Evanescent Intellect

Enchant creature
Enchanted creature has "{1}{U}, {T}: Target player mills three cards."

Godhunter Octopus

Godhunter Octopus can't attack unless defending player controls an enchantment or an enchanted permanent.

Retraction Helix

Until end of turn, target creature gains "{T}: Return target nonland permanent to its owner's hand."

Memory Drain

Vaporkin

Flying
Vaporkin can block only creatures with flying.

Crypsis

Target creature you control gains protection from creatures your opponents control until end of turn. Untap it.

Omenspeaker

When Omenspeaker enters the battlefield, scry 2. (Look at the top two cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.)

Nyxborn Seaguard

Fate Foretold

Enchant creature
When Fate Foretold enters the battlefield, draw a card.
When enchanted creature dies, its controller draws a card.

Triton Shorethief

Thirst for Meaning

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

Omen of the Sea

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

Nullify

Triton Waverider

Constellation — Whenever an enchantment enters the battlefield under your control, Triton Waverider gains flying until end of turn.

Prescient Chimera

Flying
Whenever you cast an instant or sorcery spell, scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Floodtide Serpent

Floodtide Serpent can't attack unless you return an enchantment you control to its owner's hand. (This cost is paid as attackers are declared.)

Stern Dismissal

Return target creature or enchantment an opponent controls to its owner's hand.

Chain to Memory

Target creature gets -4/-0 until end of turn. Scry 2.

Wavecrash Triton

Heroic — Whenever you cast a spell that targets Wavecrash Triton, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Sudden Storm

Tap up to two target creatures. Those creatures don't untap during their controllers' next untap steps. Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Stymied Hopes

Counter target spell unless its controller pays {1}. Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Cloaked Siren

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

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 of your library.)

Font of Fortunes

{1}{U}, Sacrifice Font of Fortunes: Draw two cards.

Naiad of Hidden Coves

As long as it's not your turn, spells you cast cost {1} less to cast.

Annul

Counter target artifact or enchantment spell.

Benthic Giant

Hexproof (This creature can't be the target of spells or abilities your opponents control.)

Lost in a Labyrinth

Target creature gets -3/-0 until end of turn. Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Witness of Tomorrows

Pin to the Earth

Enchant creature
Enchanted creature gets -6/-0.

Divination

Elite Instructor

When Elite Instructor enters the battlefield, draw a card, then discard a card.

Stratus Walk

Enchant creature
When Stratus Walk enters the battlefield, draw a card.
Enchanted creature has flying. (It can't be blocked except by creatures with flying or reach.)
Enchanted creature can block only creatures with flying.

Riptide Turtle

Triton Shorestalker

Sleep of the Dead

Tap target creature. It doesn't untap during its controller's next untap step.
Escape—{2}{U}, Exile three other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Deepwater Hypnotist

Inspired — Whenever Deepwater Hypnotist becomes untapped, target creature an opponent controls gets -3/-0 until end of turn.

Eidolon of Philosophy

{6}{U}, Sacrifice Eidolon of Philosophy: Draw three cards.

Crackling Triton

{2}{R}, Sacrifice Crackling Triton: It deals 2 damage to any target.

Nimbus Naiad

Bestow {4}{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.)
Flying
Enchanted creature gets +2/+2 and has flying.

Thassa's Devourer

Constellation — Whenever Thassa's Devourer or another enchantment enters the battlefield under your control, target player mills two cards.

Rise of Eagles

Create two 2/2 blue Bird enchantment creature tokens with flying. Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Brine Giant

This spell costs {1} less to cast for each enchantment you control.

Griptide

Put target creature on top of its owner's library.

Starlit Mantle

Flash
Enchant creature you control
When Starlit Mantle enters the battlefield, enchanted creature gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)
Enchanted creature gets +1/+1.

Sigiled Starfish

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

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 of your library.)

Coastline Chimera

Flying
{1}{W}: Coastline Chimera can block an additional creature this 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.

War-Wing Siren

Flying
Heroic — Whenever you cast a spell that targets War-Wing Siren, put a +1/+1 counter on War-Wing Siren.

Nyxborn Triton

Bestow {4}{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.)
Enchanted creature gets +2/+3.