Brinebarrow Intruder

Flash
When this creature enters, target creature an opponent controls gets -2/-0 until end of turn.

Threnody Singer

Flash
Flying
When this creature enters, 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.)

Undersea Invader

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

Riptide Turtle

Spectral Sailor

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