Protection from red Elemental creatures you control get +1/+1. When Master of Waves enters the battlefield, create a number of 1/0 blue Elemental creature tokens equal to your devotion to blue.
Put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it. If you control a Merfolk, draw a card.
"You may want to hold your breath." —Kellyg, merrow aquitect
Enchant creature When Claustrophobia enters the battlefield, tap enchanted creature. Enchanted creature doesn't untap during its controller's untap step.
You may cast Harbinger of the Tides as though it had flash if you pay {2} more to cast it. When Harbinger of the Tides enters the battlefield, you may return target tapped creature an opponent controls to its owner's hand.
Flying When Merfolk Wayfinder enters the battlefield, reveal the top three cards of your library. Put all Island cards revealed this way into your hand and the rest on the bottom of your library in any order.
When Tidal Courier enters the battlefield, reveal the top four cards of your library. Put all Merfolk cards revealed this way into your hand and the rest on the bottom of your library in any order. {3}{U}: Tidal Courier gains flying until end of turn.
When Tidebinder Mage enters the battlefield, tap target red or green creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control Tidebinder Mage.
Up to two target creatures each get +0/+3 until end of turn. Untap those creatures. At this turn's next end of combat, tap each creature that was blocked by one of those creatures this turn and it doesn't untap during its controller's next untap step.