Fleet Swallower

Whenever Fleet Swallower attacks, target player mills half their library, rounded up.

Vineshaper Mystic

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

Headwater Sentries

Tempest Caller

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

Air Elemental

Jace's Sentinel

As long as you control a Jace planeswalker, Jace's Sentinel gets +1/+0 and can't be blocked.

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.

Jade Guardian

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
When Jade Guardian enters, put a +1/+1 counter on target Merfolk you control.

Forest

Woodland Stream

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

Jungle Delver

{3}{G}: Put a +1/+1 counter on Jungle Delver.

Island

Deeproot Warrior

Whenever Deeproot Warrior becomes blocked, it gets +1/+1 until end of turn.

Shaper Apprentice

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

Grasping Current

Return up to two target creatures to their owner's hand.
Search your library and/or graveyard for a card named Jace, Ingenious Mind-Mage, reveal it, and put it into your hand. If you searched your library this way, shuffle.

River Heralds' Boon

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

Castaway's Despair

Enchant creature
When Castaway's Despair enters, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.

Kumena's Speaker

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

Herald of Secret Streams

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

Jace, Ingenious Mind-Mage

+1: Draw a card.
+1: Untap all creatures you control.
−9: Gain control of up to three target creatures.