Throwing a fish back into the water is part of Fortnite's Week 12 challenges. After the holiday-themed challenges of Week 11, Epic Games seem to be going for a more relaxed setting with most of the quests being set around fishing.
This Epic challenge especially sticks with the fishing theme - quite literally.
How to Throw a Fish Back Into the Water: Fortnite
Throwing a fish into the water is as easy as releasing it from your inventory. And that's it. Obviously, you actually need a fish first to complete this challenge, so you might want to pick up a fishing rod. Just make sure that when you do release the fish that its over the water.