The best calorie burn comes from scuba diving. For this one, you will definitely need to buy or rent equipment, plus you will need to shell out for training. The training teaches you how to dive and how to be safe while diving, and also gives you the coveted air card. Most dive shops will insist on seeing your air card before filling your tanks or renting you theirs. Scuba diving, also called skin diving, will burn up to 1,020 calories per hour if you are moving through the water quickly. Even a leisurely dive will burn about 408 calories per hour.
Swimming is a great way to burn calories, get in some cardiovascular work, and tone your entire body. It's low impact, so swimming laps is a great choice if you are overweight or have joint problems. For the best calorie burn, shoot for a fast crawl stroke (about 75 yards per minute) or just do the butterfly stroke. Both will burn about 680 calories per hour. Breaststroke and fast laps of any stroke will net you about 612 calories per hour, and even a lazy freestyle swim will expend 408 calories per hour. Swimming in open water instead of a pool will burn a few more calories as well.
If you enjoy rowing, canoeing, or kayaking, all will help you burn calories. Vigorous canoeing, at greater than 6 miles per hour, burns 748 calories. If you have to do any portaging, that's another 408 calories per hour. If you row in a scull or even a dragon boat, you will also burn about 748 calories an hour. Kayaking burns about 272 calories per hour. All the rowing activities have other benefits as well, as they work to build your upper body, shoulders, upper back, and core.
If you just want to hang out in the water and chat, try treading water while you do so. Treading water quickly will burn 612 calories while moderate treading will burn 204 calories. Just keep your arms and legs moving and you can talk all day and burn calories at the same time. Or spend time water jogging with a friend in shallow water and burn 476 calories an hour. Water aerobics is another good way to be social and get in some exercise. You'll burn 204 calories in a one hour class.
If team sports are your thing, try water polo. You can burn 612 calories in an hour of active play. Another team sport is water volleyball but you'll only burn 136 calories in an hour of play.
As with any sport, the number of calories burned will change with your weight, your level of activity, and your level of fitness. As a general rule, the more you weigh the more calories you will burn. Regardless of your level of fitness, working at your hardest level is going to burn more calories than any easy workout.