This is the same as Land Race but with waterborne craft. You have to race through checkpoints as fast as you can. For this challenge, you can use any vehicle of the same type as long as you unlocked them. If you're having problems getting the best time with the default vehicle, you can always try the challenge again with a different vehicle.
Unlocked by: Liberate Babica
The default ship is fast enough to get 5-gears but it will be a very close deal. Even if you don't try your best, you're guaranteed to get at least 4 gears at least. If you want to 5-gear this challenge, you can do so after unlocking the nitrous upgrade for sea vehicles.