Well, this is quite possibly the easiest challenge you'll come across in Fortnite.
However, don't be fooled. While there's a ton of rivers and lakes nearby, the challenge actually wants you to be *in* Lazy Lake.
That means there are only two spots you can complete this challenge — in a couple of swimming pools.
Where to swim in Lazy Lake
These pools are located in the south of the city, in backyards. All you need to do is jump in and move around a bit, and the task should be complete.
It's not known whether you can swim in the river alongside the town, but if you're in the area, it's worth a try!