Island

Winter Orb

As long as Winter Orb is untapped, players can't untap more than one land during their untap steps.

Lord of Atlantis

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

Wonder

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

Rootwater Thief

{U}: Rootwater Thief gains flying until end of turn.
Whenever Rootwater Thief deals combat damage to a player, you may pay {2}. If you do, search that player's library for a card and exile it, then the player shuffles.

Merfolk of the Pearl Trident

Merfolk Looter

Daze

You may return an Island you control to its owner's hand rather than pay this spell's mana cost.
Counter target spell unless its controller pays {1}.

Tidal Warrior

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

Vodalian Merchant

When Vodalian Merchant enters, draw a card, then discard a card.

Counterspell

Gush

You may return two Islands you control to their owner's hand rather than pay this spell's mana cost.
Draw two cards.