Wonder

Flying
As long as Wonder is in your graveyard and you control an Island, creatures you control have flying.

Counterspell

Tidal Kraken

Cleric of Chill Depths

Whenever Cleric of Chill Depths blocks a creature, that creature doesn't untap during its controller's next untap step.

Merfolk of the Pearl Trident

Whelming Wave

Return all creatures to their owners' hands except for Krakens, Leviathans, Octopuses, and Serpents.

Kraken Hatchling

Solemn Simulacrum

When this creature enters, you may search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.
When this creature dies, you may draw a card.

Island

Sky Diamond

Sleep

Tap all creatures target player controls. Those creatures don't untap during that player's next untap step.

Unsummon

Return target creature to its owner's hand.

Crown of Ascension

Enchant creature
Enchanted creature has flying.
Sacrifice Crown of Ascension: Enchanted creature and other creatures that share a creature type with it gain flying until end of turn.

Coral Commando

Overflowing Insight

Scourge of Fleets

When Scourge of Fleets enters, return each creature your opponents control with toughness X or less to its owner's hand, where X is the number of Islands you control.

Library Larcenist

Whenever Library Larcenist attacks, draw a card.

Tidal Warrior

{T}: Target land becomes an Island until end of turn.

Master of the Pearl Trident

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)