Tishana's Tidebinder

Flash
When Tishana's Tidebinder enters the battlefield, counter up to one target activated or triggered ability. If an ability of an artifact, creature, or planeswalker is countered this way, that permanent loses all abilities for as long as Tishana's Tidebinder remains on the battlefield. (Mana abilities can't be targeted.)

Subtlety

Flash
Flying
When Subtlety enters the battlefield, choose up to one target creature spell or planeswalker spell. Its owner puts it on the top or bottom of their library.
Evoke—Exile a blue card from your hand.

Lord of Atlantis

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

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.

Vodalian Hexcatcher

Flash
Other Merfolk you control get +1/+1.
Sacrifice a Merfolk: Counter target noncreature spell unless its controller pays {1}.

Mutavault

{T}: Add {C}.
{1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.

Island

Dismember

({B/P} can be paid with either {B} or 2 life.)
Target creature gets -5/-5 until end of turn.

Chalice of the Void

Chalice of the Void enters the battlefield with X charge counters on it.
Whenever a player casts a spell with mana value equal to the number of charge counters on Chalice of the Void, counter that spell.

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}.)

Cavern of Souls

As Cavern of Souls enters the battlefield, choose a creature type.
{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

Harbinger of the Tides

You may cast Harbinger of the Tides as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
When Harbinger of the Tides enters the battlefield, you may return target tapped creature an opponent controls to its owner's hand.

Spreading Seas

Enchant land
When Spreading Seas enters the battlefield, draw a card.
Enchanted land is an Island.

Cursed Totem

Activated abilities of creatures can't be activated.

Minamo, School at Water's Edge

{T}: Add {U}.
{U}, {T}: Untap target legendary permanent.

Force of Negation

If it's not your turn, you may exile a blue card from your hand rather than pay this spell's mana cost.
Counter target noncreature spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Rishadan Dockhand

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
{1}, {T}: Tap target land.

Merfolk Trickster

Flash
When Merfolk Trickster enters the battlefield, tap target creature an opponent controls. It loses all abilities until end of turn.

Otawara, Soaring City

{T}: Add {U}.
Channel — {3}{U}, Discard Otawara, Soaring City: Return target artifact, creature, enchantment, or planeswalker to its owner's hand. This ability costs {1} less to activate for each legendary creature you control.

Aether Vial

At the beginning of your upkeep, you may put a charge counter on Aether Vial.
{T}: You may put a creature card with mana value equal to the number of charge counters on Aether Vial from your hand onto the battlefield.

Master of the Pearl Trident

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