Callaphe, Beloved of the Sea

Callaphe's power is equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
Creatures and enchantments you control have "Spells your opponents cast that target this permanent cost {1} more to cast."

Siren Stormtamer

Flying
{U}, Sacrifice this creature: Counter target spell or ability that targets you or a creature you control.

Island

Dive Down

Target creature you control gets +0/+3 and gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Disdainful Stroke

Counter target spell with mana value 4 or greater.

Tempest Djinn

Flying
This creature gets +1/+0 for each basic Island you control.

Damping Sphere

If a land is tapped for two or more mana, it produces {C} instead of any other type and amount.
Each spell a player casts costs {1} more to cast for each other spell that player has cast this turn.

Gadwick, the Wizened

When Gadwick enters, draw X cards.
Whenever you cast a blue spell, tap target nonland permanent an opponent controls.

Curious Obsession

Enchant creature
Enchanted creature gets +1/+1 and has "Whenever this creature deals combat damage to a player, you may draw a card."
At the beginning of your end step, if you didn't attack with a creature this turn, sacrifice this Aura.

Unsubstantiate

Return target spell or creature to its owner's hand.

Negate

Aetherize

Return all attacking creatures to their owner's hand.

Merfolk Trickster

Flash
When this creature enters, tap target creature an opponent controls. It loses all abilities until end of turn.

Tidebinder Mage

When this creature enters, 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 this creature.

Cloudfin Raptor

Flying
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Harbinger of the Tides

You may cast this card as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
When this creature enters, you may return target tapped creature an opponent controls to its owner's hand.

Wizard's Retort

This spell costs {1} less to cast if you control a Wizard.
Counter target spell.

Master of Waves

Protection from red
Elemental creatures you control get +1/+1.
When this creature enters, create a number of 1/0 blue Elemental creature tokens equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)