Triton Waverider

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

Naiad of Hidden Coves

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

Eidolon of Philosophy

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

Callaphe, Beloved of the Sea

Callaphe's power is equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
Creatures and enchantments you control have "Spells your opponents cast that target this permanent cost {1} more to cast."

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.

Shoal Kraken

Constellation — Whenever an enchantment enters the battlefield under your control, you may draw a card. If you do, discard a card.

Glimpse of Freedom

Draw a card.
Escape—{2}{U}, Exile five other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Memory Drain

Nyx Lotus

Nyx Lotus enters the battlefield tapped.
{T}: Choose a color. Add an amount of mana of that color equal to your devotion to that color. (Your devotion to a color is the number of mana symbols of that color in the mana costs of permanents you control.)

Thaumaturge's Familiar

Flying
When Thaumaturge's Familiar enters the battlefield, scry 1.

Nyxborn Seaguard

Thirst for Meaning

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

Threnody Singer

Flash
Flying
When Threnody Singer enters the battlefield, target creature an opponent controls gets -X/-0 until end of turn, where X is your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Island

Sage of Mysteries

Constellation — Whenever an enchantment enters the battlefield under your control, target player mills two cards.

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.)

Ichthyomorphosis

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

Altar of the Pantheon

Your devotion to each color and each combination of colors is increased by one.
{T}: Add one mana of any color. If you control a God, a Demigod, or a legendary enchantment, you gain 1 life.

Unknown Shores

{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Stern Dismissal

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

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.

Sweet Oblivion

Target player mills four cards.
Escape—{3}{U}, Exile four other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Towering-Wave Mystic

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

Stinging Lionfish

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

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.

Witness of Tomorrows

Thassa, Deep-Dwelling

Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature.
At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control.
{3}{U}: Tap another target creature.