Delightful Discovery

This spell costs {1} less to cast for each spell your opponents have cast this turn.
Scry 2, then draw two cards. (To 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.)

Chrome Prowler

Flash
When Chrome Prowler enters, tap target creature an opponent controls.

Island

Crystacean

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

Capture Sphere

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
When Capture Sphere enters, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.

Stinging Lionfish

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

Wind Strider

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

Undersea Invader

Flash (You may cast this spell any time you could cast an instant.)
Undersea Invader enters tapped.

Slimebind

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature gets -4/-0.

Thriving Isle

Thriving Isle enters tapped. As it enters, choose a color other than blue.
{T}: Add {U} or one mana of the chosen color.

Surprise!

Thryx, the Sudden Storm

Flash (You may cast this spell any time you could cast an instant.)
Flying
Spells you cast with mana value 5 or greater cost {1} less to cast and can't be countered.

Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Draw a card.

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Spectral Sailor

Flash (You may cast this spell any time you could cast an instant.)
Flying
{3}{U}: Draw a card.