Harbor Serpent

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
This creature can't attack unless there are five or more Islands on the battlefield.

Island

Deeptread Merrow

{U}: This creature gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)

Merfolk Sovereign

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.

Deep-Sea Kraken

This creature can't be blocked.
Suspend 9—{2}{U} (Rather than cast this card from your hand, you may pay {2}{U} and exile it with nine time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, you may cast it without paying its mana cost. It has haste.)
Whenever an opponent casts a spell, if this card is suspended, remove a time counter from it.

Streambed Aquitects

{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.

Excavator

{T}, Sacrifice a basic land: Target creature gains landwalk of each of the land types of the sacrificed land until end of turn. (It can't be blocked as long as defending player controls a land of any of those types.)

Mind Control

Enchant creature
You control enchanted creature.

Tidal Warrior

{T}: Target land becomes an Island until end of turn.

Cancel

Slipstream Eel

This creature can't attack unless defending player controls an Island.
Cycling {1}{U} ({1}{U}, Discard this card: Draw a card.)

Dandân

This creature can't attack unless defending player controls an Island.
When you control no Islands, sacrifice this creature.

Retreat to Coralhelm

Landfall — Whenever a land you control enters, choose one —
• You may tap or untap target creature.
• Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Spreading Seas

Enchant land
When this Aura enters, draw a card.
Enchanted land is an Island.

Pirate Ship

This creature can't attack unless defending player controls an Island.
{T}: This creature deals 1 damage to any target.
When you control no Islands, sacrifice this creature.

Tideshaper Mystic

{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.