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.

Arcane Denial

Counter target spell. Its controller may draw up to two cards at the beginning of the next turn's upkeep.
You draw a card at the beginning of the next turn's upkeep.

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}.

Lord of Atlantis

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

Boomerang

Return target permanent to its owner's hand.

Tidal Warrior

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

Waterfront Bouncer

{U}, {T}, Discard a card: Return target creature to its owner's hand.

Rootwater Diver

{T}, Sacrifice Rootwater Diver: Return target artifact card from your graveyard to your hand.

Counterspell

Merfolk Traders

When Merfolk Traders enters, draw a card, then discard a card.

Foil

You may discard an Island card and another card rather than pay this spell's mana cost.
Counter target spell.