In one sense, SCUBA diving seems like incredible exercise: You strap 70-some pounds to your body, swim for an hour or so, then haul yourself back up a ladder and onto your boat. Most divers, though, will admit they're not doing all that much, fitness-wise, while they're down there. "I do get a chuckle out of calling it a sport," says Dive Operations Supervisor Mick Maher of Reef Divers in the Cayman Islands, voted best overall destination in the Caribbean and the Atlantic by SCUBA Diving magazine. "It's usually more about relaxation and taking your time in the water."
So why are you so wiped out afterward? "A lot of it has to do with your body expending energy to fight the cooling effects of the water's lower temperatures," says Maher. Your lungs also have to readjust to normal air after breathing compressed air, which is higher in oxygen, he adds. Add in the fact that the sun's beating down on you and you're probably at least a little dehydrated, and you've got a perfect recipe for an afternoon nap.
Of course, diving can be as much of a workout as you want to make it. The 2011 Compendium of Physical Activities (a database that measures the energy cost of sports and activities by metabolic equivalents, or METs) gives SCUBA a MET ranking of 7.0, putting it on par with ice skating, very brisk walking, and playing casual soccer. That means that a 180-pound male will burn about 600 calories in an hour of diving—although that amount can vary considerably based on how much he moves around underwater and whether he swims with or against the current. (Extreme divers who recover heavy fossils or hunt and spear fish while they're underwater, for example, clearly get more of a workout.)
If diving really is doing a number on you, physically, talk to your doctor. In 2013, the American College of Sports Medicine presented a seminar about SCUBA safety, noting that the cardiovascular demands of diving can make it dangerous for people who are overweight or obese, or who have underlying health conditions. Losing weight and improving your overall fitness level can make you a better diver, says Maher, and help you enjoy your experience even more.
Bottom line: SCUBA diving may not be as effective as an hour on the treadmill, but it's certainly more appealing. And hey, it's a better workout than staying on the boat and drinking beer! Enjoy it for what it is: a fantastic hobby that happens to burn plenty of calories—but don't let it become a replacement for cardio and strength training.