Because you have electrical super powers, stepping in pools of water will obviously fry anything in it. For this trophy, you'll have to kill an enemy or civilian while they're in a body of water by stepping in it in order to unlock it. Shooting lightning when an enemy’s in the water does not count.
You can earn this during any story mission or side mission that takes place near or inside the swamp. However, you will have an easier time in earning this, and most likely by complete accident when you've unlocked the second island, Flood Town.