Jokulmorder

Trample
This creature enters tapped.
When this creature enters, sacrifice it unless you sacrifice five lands.
This creature doesn't untap during your untap step.
Whenever you play an Island, you may untap this creature.

Aboleth Spawn

Flash
Ward {2}
Probing Telepathy — Whenever a creature entering under an opponent's control causes a triggered ability of that creature to trigger, you may copy that ability. You may choose new targets for the copy.

Flooded Shoreline

{U}{U}, Return two Islands you control to their owner's hand: Return target creature to its owner's hand.

Floodgate

Defender
When this creature has flying, sacrifice it.
When this creature leaves the battlefield, it deals damage to each nonblue creature without flying equal to half the number of Islands you control, rounded down.

Fathom Seer

Morph—Return two Islands you control to their owner's hand. (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When this creature is turned face up, draw two cards.

Engulf the Shore

Return to their owners' hands all creatures with toughness less than or equal to the number of Islands you control.

Daze

You may return an Island you control to its owner's hand rather than pay this spell's mana cost.
Counter target spell unless its controller pays {1}.

Charix, the Raging Isle

Spells your opponents cast that target Charix cost {2} more to cast.
{3}: Charix gets +X/-X until end of turn, where X is the number of Islands you control.

Biolume Serpent

Sacrifice two Islands: This creature can't be blocked this turn.

Biolume Egg

Defender
When this creature enters, scry 2.
When you sacrifice this creature, return it to the battlefield transformed under its owner's control at the beginning of the next end step.

Biolume Egg // Biolume Serpent

Flow of Knowledge

Draw a card for each Island you control, then discard two cards.

High Tide

Until end of turn, whenever a player taps an Island for mana, that player adds an additional {U}.

Ensnare

You may return two Islands you control to their owner's hand rather than pay this spell's mana cost.
Tap all creatures.

Coral Reef

This enchantment enters with four polyp counters on it.
Sacrifice an Island: Put two polyp counters on this enchantment.
{U}, Tap an untapped blue creature you control, Remove a polyp counter from this enchantment: Put a +0/+1 counter on target creature.