Unstable Mutation

Enchant creature
Enchanted creature gets +3/+3.
At the beginning of the upkeep of enchanted creature's controller, put a -1/-1 counter on that creature.

Ghost Ship

Power Leak

Enchant enchantment
At the beginning of the upkeep of enchanted enchantment's controller, that player may pay any amount of mana. Power Leak deals 2 damage to that player. Prevent X of that damage, where X is the amount of mana that player paid this way.

Flight

Enchant creature
Enchanted creature has flying.

Counterspell

Prodigal Sorcerer

{T}: Prodigal Sorcerer deals 1 damage to any target.

Sunken City

At the beginning of your upkeep, sacrifice Sunken City unless you pay {U}{U}.
Blue creatures get +1/+1.

Flood

{U}{U}: Tap target creature without flying.

Island

Merfolk of the Pearl Trident

Air Elemental

Wall of Air

Defender, flying (This creature can't attack, and it can block creatures with flying.)