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}.

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.

Lord of Atlantis

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

Island

Tidal Warrior

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

Wonder

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

Merfolk Looter

Counterspell

Merfolk of the Pearl Trident

Vodalian Merchant

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

Winter Orb

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

Gush

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