Lord of Atlantis

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

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.

Opposition

Tap an untapped creature you control: Tap target artifact, creature, or land.

Island

Teferi's Response

Counter target spell or ability an opponent controls that targets a land you control. If a permanent's ability is countered this way, destroy that permanent.
Draw two cards.

Vodalian Merchant

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

Prodigal Sorcerer

{T}: Prodigal Sorcerer deals 1 damage to any target.

Thwart

You may return three Islands you control to their owner's hand rather than pay this spell's mana cost.
Counter target spell.

Waterfront Bouncer

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

Hibernation

Return all green permanents to their owners' hands.

Misdirection

You may exile a blue card from your hand rather than pay this spell's mana cost.
Change the target of target spell with a single target.

Static Orb

As long as Static Orb is untapped, players can't untap more than two permanents during their untap steps.

Darting Merfolk

{U}: Return Darting Merfolk to its owner's hand.

Rushing River

Kicker—Sacrifice a land. (You may sacrifice a land in addition to any other costs as you cast this spell.)
Return target nonland permanent to its owner's hand. If this spell was kicked, return another target nonland permanent to its owner's hand.

Merfolk Looter

Merfolk of the Pearl Trident

Wash Out

Return all permanents of the color of your choice to their owners' hands.

Mana Maze

Players can't cast spells that share a color with the spell most recently cast this turn.