Tidal Warrior

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

Island

Rootwater Hunter

{T}: Rootwater Hunter deals 1 damage to any target.

Feldon's Cane

{T}, Exile this artifact: Shuffle your graveyard into your library.

Maze of Shadows

{T}: Add {C}.
{T}: Untap target attacking creature with shadow. Prevent all combat damage that would be dealt to and dealt by that creature this turn.

Telepathy

Your opponents play with their hands revealed.

Mystic Compass

{1}, {T}: Target land becomes the basic land type of your choice until end of turn.

Rootwater Mystic

{1}{U}: Look at the top card of target player's library.

Spellbook

Sunken City

At the beginning of your upkeep, sacrifice Sunken City unless you pay {U}{U}.
Blue creatures get +1/+1.

Douse

Benthic Explorers

{T}, Untap a tapped land an opponent controls: Add one mana of any type that land could produce.

Seasinger

When you control no Islands, sacrifice Seasinger.
You may choose not to untap Seasinger during your untap step.
{T}: Gain control of target creature whose controller controls an Island for as long as you control Seasinger and Seasinger remains tapped.

Dissipate

Counter target spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Energy Field

Prevent all damage that would be dealt to you by sources you don't control.
When a card is put into your graveyard from anywhere, sacrifice Energy Field.

Sandbar Merfolk

Cycling {2} ({2}, Discard this card: Draw a card.)

Magical Hack

Change the text of target spell or permanent by replacing all instances of one basic land type with another. (For example, you may change "swampwalk" to "plainswalk." This effect lasts indefinitely.)

Sea Scryer

Counterspell

Lord of Atlantis

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

Psychic Venom

Enchant land
Whenever enchanted land becomes tapped, Psychic Venom deals 2 damage to that land's controller.

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.

Annul

Counter target artifact or enchantment spell.

Coral Atoll

Coral Atoll enters tapped.
When Coral Atoll enters, sacrifice it unless you return an untapped Island you control to its owner's hand.
{T}: Add {C}{U}.