Tide Shaper

Kicker {1} (You may pay an additional {1} as you cast this spell.)
When Tide Shaper enters the battlefield, if it was kicked, target land becomes an Island for as long as Tide Shaper remains on the battlefield.
Tide Shaper gets +1/+1 as long as an opponent controls an Island.

Grand Arbiter Augustin IV

White spells you cast cost {1} less to cast.
Blue spells you cast cost {1} less to cast.
Spells your opponents cast cost {1} more to cast.

Coral Fighters

Whenever Coral Fighters attacks and isn't blocked, look at the top card of defending player's library. You may put that card on the bottom of that player's library.

Ertai, Wizard Adept

Counterspell

Rootwater Commando

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)

Lord of Atlantis

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

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.

Svyelun of Sea and Sky

Svyelun of Sea and Sky has indestructible as long as you control at least two other Merfolk.
Whenever Svyelun attacks, draw a card.
Other Merfolk you control have ward {1}. (Whenever another Merfolk you control becomes the target of a spell or ability an opponent controls, counter it unless that player pays {1}.)

Vodalian Arcanist

{T}: Add {C}. Spend this mana only to cast an instant or sorcery spell.

Talrand, Sky Summoner

Whenever you cast an instant or sorcery spell, create a 2/2 blue Drake creature token with flying.

Tidal Courier

When Tidal Courier enters the battlefield, reveal the top four cards of your library. Put all Merfolk cards revealed this way into your hand and the rest on the bottom of your library in any order.
{3}{U}: Tidal Courier gains flying until end of turn.

Baral, Chief of Compliance

Instant and sorcery spells you cast cost {1} less to cast.
Whenever a spell or ability you control counters a spell, you may draw a card. If you do, discard a card.

Overtaker

{3}{U}, {T}, Discard a card: Untap target creature and gain control of it until end of turn. That creature gains haste until end of turn.

Mistform Ultimus

Mistform Ultimus is every creature type (even if this card isn't on the battlefield).

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.

Rootwater Matriarch

{T}: Gain control of target creature for as long as that creature is enchanted.

True-Name Nemesis

As True-Name Nemesis enters the battlefield, choose a player.
True-Name Nemesis has protection from the chosen player.

Empress Galina

{U}{U}, {T}: Gain control of target legendary permanent. (This effect lasts indefinitely.)