This challenge is part of the Rippley vs Sludge Overtime Mission, which ends Chapter 2 Season 1 of Fortnite. The next challenge is about swimming 50m in less than 5 seconds. To achieve that, you'll have to use the Dolphin Diving strategy. That's not all because if you swim in a calm stream, you won't generate enough speed. You'll have to use the different streams available in the map.
Streams
They are symbolized on the map by white ripples on the waterways.
Go to one of these rivers, swim in the direction of the current using the dolphin swim, and you'll complete the challenge. As usual, we advise you to favor the Team Rumble mode, so that you can reappear in case of elimination.