Ichthyomorphosis

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

Riptide Turtle

Brine Giant

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

Stern Dismissal

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

Island

Triton Waverider

Constellation — Whenever an enchantment you control enters, Triton Waverider gains flying until end of turn.

Omen of the Sea

Flash (You may cast this spell any time you could cast an instant.)
When Omen of the Sea enters, scry 2, then draw a card.
{2}{U}, Sacrifice Omen of the Sea: 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.)

Thaumaturge's Familiar

Flying
When Thaumaturge's Familiar enters, scry 1.

Nyxborn Seaguard

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.

Witness of Tomorrows

Naiad of Hidden Coves

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

Thirst for Meaning

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