Given that he'll soon reprise his role from 300 as Xerxes—the highly sculpted, practically naked Persian king who thinks he's a god—it's a good thing Rodrigo Santoro stays in great shape just for fun.

The six-foot-three, 170-pound Brazilian actor, who will star in next year's 300: Rise of an Empire, possesses the kind of lean, sinewy physique (a strong V-shaped back, a chiseled torso, and toned biceps) that you would expect to spot not at the gym but on the beach, clad in board shorts and clutching a surfboard.

"It's a workout, but it makes me feel good," Santoro says. "You're building your core by riding waves and your back and shoulders by paddling." Originally from Rio de Janeiro, Santoro, 37, started surfing when he was 17 and has continued to chase waves throughout the Americas, even as he's also catching big breaks as an actor. "I lived in Hawaii for a while doing Lost's third season, and it was paradise," he says. "I'd go surfing three times a day when I wasn't working."

Whether or not the ocean is at your disposal, you can build a tight, well-defined body fit for the beach with the right moves. When Santoro can't get in the water, he works on his upper-body and core strength with dry-land body-weight moves that help him not only perform better on the board but also look better off it. "Flexibility and mobility, combined with strength and stability—especially through the core—are what surfers need," says Joey Wolfe of Paradigm Sport, who trains pro surfers like American sensation Nat Young and helped create the following workout. You don't have to do it on the sand, but you might want to. In addition to achieving enviable abdominal and upper-body definition, Santoro gets an energy boost from working out in the sun and sea air. "It's a blissful sensation," he says. Plus, "a beer just tastes better afterwards."

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The Essential Beach Workout

Tip: Begin this routine with a few minutes of easy jogging to warm up, then go through all of the exercises once. Rest for 30 seconds to a minute between moves.

1. Knee-to-Chest Burpee
Stand straight, arms at your sides. Moving rapidly, crouch down, with hands by your feet. Kick both legs behind you to a high push-up position, then return to the crouch. Stand and jump, bringing your knees to your chest. Do 2 or 3 sets of 8 to 12 reps.

2. Plank With Diagonals
Begin in plank position, elbows on the ground. Keeping your core firm, lift one arm straight up by your head while raising the opposite leg. Lower with control and alternate sides until you've done 12 reps.

3. Spider-Man Push-up
Start in high plank position. As you lower to the floor, swing your right knee out sideways, reaching for the right elbow. Return to starting position and repeat on your left side. Alternate legs for 30 seconds.

4. Lateral Lunge
Stand with your hands raised above your head. Keep one leg straight and step wide to the side, bending that knee and holding for a beat. Return to standing while rotating the body 180 degrees. Repeat the movement with the other leg. Complete 4 lunges on each side.
(Surfboard not required.)

5. Extended Side Angle
Lunge forward with your left leg while splaying your right foot out to a 45-degree angle. Twist your trunk open, lower your left hand to the sand, and raise your right hand to the sky. Hold for a count of 8, then repeat on the other side.

6. Single-Leg Squat
Hold your arms straight out in front of you and balance on your left foot, slowly bending at the knee until you reach a seated position. Lift the left leg until thighs are parallel and hold for a count of 8. Do 2 reps on each leg.

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Kettlebell Moves to Try

You only need to bring one weight to the beach to get extra definition without extra bulk.

1. Kettlebell Thruster
Start in a baseball catcher's position and hold the round part of the kettlebell (try a 35-pounder) at your Adam's apple. Spread your knees out at a 45-degree angle and stand up, lifting the kettlebell over your head. Return to the squat position while lowering the bell. Do 3 sets of 6 reps.

2. Waiter Walk With Weight
Stand straight, holding the kettlebell with one arm. Tighten your abs and keep your shoulders down while lifting the kettlebell over your head until your elbow is straight. Walk at a normal pace without arching your back for 15 steps, turn around, and return. Do 2 reps with each arm.

3. Halo (right)
Stand straight and hold the kettlebell in front of your chin. Maintain an upright trunk, and bring it in a circle behind your head to the base of your skull and return to the front. Repeat 4 to 6 times in each direction, clockwise and counterclockwise.

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7 Days to a Beach-Ready Body

You can't transform your entire physique in a week, but you can firm up and feel better about going shirtless. "There are ways to flush out fluids and look as defined as possible," says Rob Hoflund, a trainer at Crunch in Los Angeles. Here's his seven-day plan.

DAY 1: Cut out all alcohol. The empty calories and sugars are a total waste.

DAY 2: No more refined carbs (white bread, pasta). High-protein carbs (oats, quinoa) are still okay.

DAY 3: Perform a high-intensity interval-training routine (on top of your regular workout) each morning to boost your metabolism.

DAY 4: Hit the sauna to start reducing bloat—your body needs a few days to flush out the fluids. Stay hydrated, but don't drink unless thirsty.

DAY 5: Add several servings of watermelon a day—it's a natural diuretic full of vitamin C.

DAY 6: The biggest muscle groups burn the most calories: Do as many squats and lunges as you can stand today.

DAY 7: Delts, traps, and pecs are the most prominent muscles, so work those with kettlebell halos and push-ups right before the beach, whether you plan to carry a surfboard or just a Kindle.

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Surf-Inspired Classes for the City Dweller

Surfset fitness classes incorporate a training machine that replicates the muscle-defining motions of paddling, hopping up, and riding waves. Find a class near you at Crunch locations in New York City, Miami Beach, and Burbank also offer the class.

Paddleboard yoga classes build many of the same muscle groups—shoulders, oblique abs, mid-to-upper back, and legs—and are offered in most major cities.

High-intensity interval training in a pool mimics the motions of paddling out to a wave and can help you sculpt and define your shoulders and upper back without a board. Equinox offers aqua boot camps.