To complete one of the Plunge! challenges, you will need to complete a race against the clock in Lazy Lake. So you have to go there to start the timed challenge. You will then only have to swim to complete the course in time to validate the challenge.
Map made with Lootlake.info
For this challenge, you will have to go to one of the two times specified by the challenge, and swim to complete the path in time.
As a reminder, it's easier to complete this challenge in Team Rumble, then with respawning, you won't have to worry about dying.