This Fortnite challenge is part of the TNTina’s Trial Challenges, the second agent that Chapter 2 Season 2 focuses on. To complete it, you'll have to fish with explosives. You may not know this, but there isn't one, nor two, but three ways to fish in Fortnite. You can use a fishing rod, a harpoon, or explosives.
For your fish hunt, you'll need an explosive item, such as Remote Explosives, Grenades or a Rocket Launcher for example. Find a spot of water and follow it until you come across an eddy. These small circles causing ripples are actually school of fish. Throw your explosives over the area and any fish or items present will rise to the surface.
Be careful: when you fish, you become an easy target for your opponents. Make sure you are alone in the area before you start anything.