You already: Swim laps using the crawl (204 calories for 20 minutes of moderate or light effort).
You should also: Try the breaststroke (305 calories for 24 minutes). It may surprise you to hear that you can actually use up more energy while keeping your hair dry with this uncomplicated stroke. You'll only need to tack on a few more laps—or four minutes—to burn 100 extra calories in the pool.
Bonus tip: If you're really pressed for time, it may be worth learning to gracefully execute the butterfly stroke (280 calories for 20 minutes).