This is one of the challenges for Chaper 2 Season 5 Week 6 of Fortnite.
You'll be asked to blow up three fishing holes at Lazy Lake, Canoe Lake, and near Steamy Stacks.
Among the areas mentioned, only Lake Canoe is a notable place not explicitly labelled on your map, while Lazy Lake and Steamy Stacks are both very easy to spot.
Where to blow up Fishing Holes
Fishing holes designate schools of fish, and as such you can spot them from a distance on the surface of the water.
Given you have to blow up these spots, you'll need explosives. Grab yourself some grenades, and chuck one within the circle.
Fishing holes disappear after being exhausted, so you'll need to find three separate ones to complete the challenge.
Also, it will often be more efficient to land directly at Lazy Lake, Lake Canoe, or near Steamy Stacks, because there's no guarantee of seeing schools of fish there later in the game if other players are around!