As the days start getting longer and the mercury is on the rise, we all may be looking for ways to cool off this summer. But rather than staying cooped up inside the air conditioning, why not get outside and get active, all while burning tons of calories? The key ingredient for fun, calorie-burning exercise: water. I'm not referring to drinking water (although drinking adequate amounts of water in the sweltering, sticky summer months is definitely important). I'm talking about the water activities that are often associated with summer.

Physical activity done in water, or water sports, burn a huge amount of calories in a short amount of time. Any time your body has to work against resistance, you'll burn extra calories, and you're encountering a great deal of resistance in the water. You'll be burning fat and improving your muscle tone because you're using all of your muscle groups at one time--your core, your legs and your arms.

Physical activities performed in the water are also low-impact, meaning they won't be as harsh on your joints as many other forms of exercise. You'll also increase your flexibility and your balance because you'll be able to perform movements with various ranges of motion. And best of all--you'll be having so much fun that you won't even realize you're exercising.

Just how great are water activities at scorching calories? Well, the number of calories you burn per hour of any physical activity will be based on your weight. For the calories burned per each activity listed below, it'll be based on a body weight of 154 pounds (70 kg). If you weigh more, you will burn more calories, and if you weigh less you will burn fewer calories.

Here is a rundown of how many calories you can burn doing an hour's worth of these water-based activities:

Canoeing (rowing with moderate effort, about 4-5.9 mph): 490 calories/hour

Diving (springboard, platform): 210 calories/hour

Kayaking: 350 calories/hour

Paddle boating: 280 calories/hour

Power boating: 175 calories/hour

Sailing (competitively): 350 calories/hour

Sailing (general, boat or board, or windsurfing): 210 calories/hour

Scuba diving: 490 calories/hour

Ski-mobiling: 490 calories/hour

Snorkeling: 350 calories/hour

Surfing (body surfing or board surfing): 210 calories/hour

Swimming (in a river, lake, or the ocean): 420 calories/hour

Swimming (for leisure): 420 calories/hour

Lap swimming (freestyle swimming, light-moderate exertion): 490 calories/hour

Lap swimming (freestyle swimming, vigorously): 700 calories/hour

Swimming (backstroke): 490 calories/hour

Swimming (breaststroke): 700 calories/hour

Swimming (butterfly): 770 calories/hour

Swimming (sidestroke): 560 calories/hour

Swimming (slow-crawl, light to moderate exertion, covering about 50 yards per minute): 560 calories/hour

Swimming (fast-crawl, vigorously, covering about 75 yards per minute): 770 calories/hour

Swimming (treading water with moderate exertion): 280 calories/hour

Swimming (treading water vigorously at a fast pace): 700 calories/hour

Synchronized swimming: 560 calories/hour

Water aerobics or water calisthenics: 280 calories/hour

Water jogging: 560 calories/hour

Water polo: 700 calories/hour

Water skiing: 420 calories/hour

Water volleyball: 210 calories/hour

Whitewater rafting (in a kayak or canoe): 350 calories/hour