What do lifeguards, surfers, and Navy SEALs have in common? They all have bodies that look perfect on the beach, you might be thinking. Their muscles are lean and taut but powerful, and their torsos taper into a V shape. There is something else they have in common, though. Their bodies aren't built by lifting iron bars or pounding paved roads. These are true beach bodies, carved by clocking time on the sand and in the water. In fact, popular adaptations of the Navy SEAL training routine are known as Hell on the Beach. Welcome to mankind's first, and still most effective, gym.

These facts aren't lost on Hollywood, where water-centric workouts are gaining serious traction. Twilight's Kellan Lutz and True Blood stud Ryan Kwanten both sculpt their cut-from-stone shapes in and around the pool and ocean—it's called a swimmer's V for a reason. Mixing in some water time with your beach routine is essential, but swimming isn't the only path to that sexy tapered midriff—it's just the quickest. No other sport builds muscles and torches fat as effectively. It is the fitness world's only legit shortcut, like a sanctioned cheat sheet to chiseldom.

The secret of swimming's transformative power is simple. A single stroke simultaneously fires 12 major muscle groups from head to toe, combining in one motion more muscles than you'd use running a marathon or rowing the Thames. It's also the planet's lowest-impact sport. And because swimming forces you to regulate your breathing, it reduces stress as effectively as yoga.

Sure, the pool or a lake is a good place to start, but that's a little bit like jogging on a treadmill instead of on the open road. A flat, steady belt is easier than a trail, where dips and hills keep your body guessing, forcing it to work harder. In other words, to get the most from a swim, hit the ocean. If it seems intimidating, make it easier by adding fins and a snorkel.

All you need to do is wade out past the break and create an imaginary lap lane. Make it parallel to the beach. Swim for about 20 to 30 yards. Stop, catch your breath, make sure you're still flanking the beach, and repeat. Try for 10 of those and increase your reps each time you're out. It's all you need to build the perfect beach body.

Taking to the water is actually part of what makes us human. Many biologists and anthropologists believe in something known as the aquatic-ape theory: that our ancient ancestors (unlike other primates) waded and swam to survive. They speculate that this pseudo-amphibian past is responsible for us having hooded noses (enabling us to dive), lowered larynxes (allowing us to hold our breath), and nearly hairless bodies (reducing friction while swimming), and why we can't live without a few key nutrients found primarily in the ocean (iodine and omega-3s). Still, despite these adaptations, we have obvious limitations after diving in. We have arms and legs that are at once our best friends and worst enemies; they give us power and propel us but cause tremendous resistance. These have long been the two opposing principles of swimming: The sport's essence is the eternal quest for more propulsion and less drag.

The shoulder roll, the flutter kick, and the high elbow catch in the front crawl—also known as the freestyle stroke—are the three secrets to counterbalancing propulsion and drag and toning all those envy-inducing muscles. First, you want to shoot your hand into the water at shoulder-width and launch it straight ahead of you (never cross in toward your body's plane). At the same time, drive your same-side hip forward and roll slightly so your armpit and lat muscle are facing the seafloor before starting your pull—your kick will steady you.

Swim science is changing the way we follow through on our stroke. "It feels powerful and natural to dig deep, drop your elbow, and pull under your body," says Gary Hall Sr., an Olympic medalist and the director and stroke guru for the Race Club, a swim camp in Tavernier, Florida. "It's what's been taught for years, but it's wrong." Experts have found it's a far less effective way to power through the water. Instead, employ a new technique called the "high elbow catch." After the shoulder roll, plant, or freeze, the elbow of your stroke arm and swing your hand and forearm straight under your locked elbow. It feels awkward at first, but it's highly effective (Michael Phelps and Ryan Lochte recently adopted it), and, best of all, it will activate even more of your key, beach-body muscles.

The trick to the freestyle flutter kick is limiting how much your knees bend. The more pronounced the bend, the more your legs, hips, and torso sink, which creates molasses-like drag. Instead, start your kick from the hips with only a slight knee bend (as if your feet were inside a bucket). Think small—quick, short snaps—and you'll see big results.

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The Best New Gear for Attacking the Ocean
The Best Water Bottle for the Beach: Safe Summer Hydration
How to Turn the Ocean Into Your Gym