Sealock Monster

This creature can't attack unless defending player controls an Island.
{5}{U}{U}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, target land becomes an Island in addition to its other types.

Shipbreaker Kraken

{6}{U}{U}: Monstrosity 4. (If this creature isn't monstrous, put four +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, tap up to four target creatures. Those creatures don't untap during their controllers' untap steps for as long as you control this creature.

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.)

Swan Song

Bird

Counter target enchantment, instant, or sorcery spell. Its controller creates a 2/2 blue Bird creature token with flying.

Thassa, God of the Sea

Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
At the beginning of your upkeep, scry 1.
{1}{U}: Target creature you control can't be blocked this turn.

Thassa's Bounty

Draw three cards. Target player mills three cards.

Thassa's Emissary

Bestow {5}{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.)
Whenever this creature or enchanted creature deals combat damage to a player, draw a card.
Enchanted creature gets +3/+3.

Triton Fortune Hunter

Heroic — Whenever you cast a spell that targets this creature, draw a card.

Decorated Griffin

Flying
{1}{W}: Prevent the next 1 combat damage that would be dealt to you this turn.

Triton Shorethief

Triton Tactics

Up to two target creatures each get +0/+3 until end of turn. Untap those creatures. At this turn's next end of combat, tap each creature that was blocked by one of those creatures this turn and it doesn't untap during its controller's next untap step.

Vaporkin

Flying
This creature can block only creatures with 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.)

Wavecrash Triton

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

Harpy

Abhorrent Overlord

Flying
When this creature enters, create a number of 1/1 black Harpy creature tokens with flying equal to your devotion to black. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)
At the beginning of your upkeep, sacrifice a creature.

Agent of the Fates

Deathtouch
Heroic — Whenever you cast a spell that targets this creature, each opponent sacrifices a creature of their choice.

Theros