Kumena's Speaker

Kumena's Speaker gets +1/+1 as long as you control another Merfolk or an Island.

Island

River Sneak

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

Simic Guildgate

Simic Guildgate enters tapped.
{T}: Add {G} or {U}.

Watertrap Weaver

When Watertrap Weaver enters, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Merfolk Mistbinder

Kopala, Warden of Waves

Spells your opponents cast that target a Merfolk you control cost {2} more to cast.
Abilities your opponents activate that target a Merfolk you control cost {2} more to activate.

Herald of Secret Streams

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

Silvergill Adept

As an additional cost to cast this spell, reveal a Merfolk card from your hand or pay {3}.
When Silvergill Adept enters, draw a card.

Seafloor Oracle

Whenever a Merfolk you control deals combat damage to a player, draw a card.

Tempest Caller

When Tempest Caller enters, tap all creatures target opponent controls.

Vineshaper Mystic

When Vineshaper Mystic enters, put a +1/+1 counter on each of up to two target Merfolk you control.

Woodland Stream

Woodland Stream enters tapped.
{T}: Add {G} or {U}.

Shaper Apprentice

Shaper Apprentice has flying as long as you control another Merfolk.

Forest

River Heralds' Boon

Put a +1/+1 counter on target creature and a +1/+1 counter on up to one target Merfolk.

Merfolk Branchwalker

When Merfolk Branchwalker 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.)

Shapers of Nature

{3}{G}: Put a +1/+1 counter on target creature.
{2}{U}, Remove a +1/+1 counter from a creature you control: Draw a card.

Merfolk Skydiver

Flying
When Merfolk Skydiver enters, put a +1/+1 counter on target creature you control.
{3}{G}{U}: Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)