Bright Reprisal

Destroy target attacking creature.
Draw a card.

River Sneak

This creature can't be blocked.
Whenever another Merfolk you control enters, this creature gets +1/+1 until end of turn.

River's Rebuke

Return all nonland permanents target player controls to their owner's hand.

Run Aground

Put target artifact or creature on top of its owner's library.

Treasure

Sailor of Means

When this creature enters, create a Treasure token. (It's an artifact with "{T}, Sacrifice this token: Add one mana of any color.")

Ixalan Checklist

Search for Azcanta // Azcanta, the Sunken Ruin

The Sunken Ruin houses all the learning of a long-forgotten age.

Search for Azcanta

At the beginning of your upkeep, surveil 1. Then if you have seven or more cards in your graveyard, you may transform Search for Azcanta. (Look at the top card of your library. You may put that card into your graveyard.)

Azcanta, the Sunken Ruin

(Transforms from Search for Azcanta.)
{T}: Add {U}.
{2}{U}, {T}: Look at the top four cards of your library. You may reveal a noncreature, nonland card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Shaper Apprentice

This creature has flying as long as you control another Merfolk.

Shipwreck Looter

Raid — When this creature enters, if you attacked this turn, you may draw a card. If you do, discard a card.

Shore Keeper

{7}{U}, {T}, Sacrifice this creature: Draw three cards.

Siren Lookout

Flying
When this creature enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

Siren Stormtamer

Flying
{U}, Sacrifice this creature: Counter target spell or ability that targets you or a creature you control.

Demystify

Siren's Ruse

Exile target creature you control, then return that card to the battlefield under its owner's control. If a Pirate was exiled this way, draw a card.

Spell Pierce

Counter target noncreature spell unless its controller pays {2}.

Spell Swindle

Counter target spell. Create X Treasure tokens, where X is that spell's mana value. (They're artifacts with "{T}, Sacrifice this token: Add one mana of any color.")

Storm Fleet Aerialist

Flying
Raid — This creature enters with a +1/+1 counter on it if you attacked this turn.