This challenge is part of the Meowscles Mischief Challenges, the third agent Fortnite Chapter 2 Season 2 focuses on. You'll have to catch a weapon, a can, and a fish. Note that you can fish in different ways — harpoon, fishing rod or explosives.
You may not know it, but the waters of Apollo Island water are full of surprises, and fish isn't the only thing to catch. In addition to small fry, flopper, slurpfish or the mythic goldfish, you can find ammunition, weapons or even... rusty cans!
To complete in your challenge, you'll need to be patient, go fishing and hope to find the famous items you're looking for. Team Rumble seems to be preferable, as you'll be less likely to be attacked and you'll have the possibility of respawning.