Have

Sigiled Starfish

{T}: Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Exotic Pets

Create two 1/1 blue Fish creature tokens with "This creature can't be blocked." Then for each kind of counter among creatures you control, put a counter of that kind on either of those tokens.

Island

Keep Safe

Counter target spell that targets a permanent you control.
Draw a card.

Victory's Envoy

At the beginning of your upkeep, put a +1/+1 counter on each other creature you control.

Brave the Sands

Creatures you control have vigilance.
Each creature you control can block an additional creature each combat.

Treasure Cove

(Transforms from Treasure Map.)
{T}: Add {C}.
{T}, Sacrifice a Treasure: Draw a card.

Treasure Map

{1}, {T}: Scry 1. Put a landmark counter on Treasure Map. Then if there are three or more landmark counters on it, remove those counters, transform Treasure Map, and create three Treasure tokens. (They're artifacts with "{T}, Sacrifice this artifact: Add one mana of any color.")

Treasure Map // Treasure Cove

The Reality Chip

You may look at the top card of your library any time.
As long as The Reality Chip is attached to a creature, you may play lands and cast spells from the top of your library.
Reconfigure {2}{U} ({2}{U}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Herald of Secret Streams

Creatures you control with +1/+1 counters on them can't be blocked.

Heraldic Banner

As this artifact enters, choose a color.
Creatures you control of the chosen color get +1/+0.
{T}: Add one mana of the chosen color.

Prairie Stream

({T}: Add {W} or {U}.)
Prairie Stream enters tapped unless you control two or more basic lands.

Planar Disruption

Enchant artifact, creature, or planeswalker
Enchanted permanent can't attack or block, and its activated abilities can't be activated.

Aquatic Incursion

When Aquatic Incursion enters, create two 1/1 blue Merfolk creature tokens with hexproof. (They can't be the targets of spells or abilities your opponents control.)
{3}{U}: Target Merfolk can't be blocked this turn.

Fabled Passage

{T}, Sacrifice Fabled Passage: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle. Then if you control four or more lands, untap that land.

Sleep with the Fishes

Enchant creature
When Sleep with the Fishes enters, tap enchanted creature and you create a 1/1 blue Fish creature token with "This creature can't be blocked."
Enchanted creature doesn't untap during its controller's untap step.

Basri's Solidarity

Put a +1/+1 counter on each creature you control.

Vodalian Hexcatcher

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

Mist-Cloaked Herald

Expendable Lackey

{1}{U}, Exile Expendable Lackey from your graveyard: Create a 1/1 blue Fish creature token with "This creature can't be blocked." Activate only as a sorcery.

Reconnaissance Mission

Whenever a creature you control deals combat damage to a player, you may draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

Stinging Lionfish

Whenever you cast your first spell during each opponent's turn, you may tap or untap target nonland permanent.

Plains

Rogue's Passage

{T}: Add {C}.
{4}, {T}: Target creature can't be blocked this turn.

Merfolk Trickster

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

Reservoir Kraken

Trample, ward {2}
At the beginning of each combat, if Reservoir Kraken is untapped, any opponent may tap an untapped creature they control. If they do, tap Reservoir Kraken and create a 1/1 blue Fish creature token with "This creature can't be blocked."