Some of the attendees are Pilates students, some are stand-up paddlers. Many have not done either sport before and are just looking for a fitness challenge that's "something different. The people in the class are of all fitness levels," says Clark, who comes from Sydney.

My class included friends Donna Cornish, 48, and Karen Demarco, 41, both new to SUPilates.

"We've been wanting to try it for a while now," says Cornish, a wine and jewellery entrepreneur. "I am attracted to it because it's done outdoors. With the weather so beautiful now, it's much better than being stuck in a gym all the time. Pilates and stand-up paddle boarding is a good combination for fitness."

Pilates, practised either on a mat or a machine called a reformer, emphasises proper breathing, correct spinal and pelvic alignment, and concentration on smooth, flowing movement. Pilates exercises help to develop long, lean muscles and a strong "core" (the muscles deep in the abdominals and closest to the spine). They also improve flexibility and posture, and are practised by professional athletes as a way to prevent injuries.

Stand-up paddle boarding packs much more action and adrenaline. Riders glide on water by standing on a board similar to a surfboard - just longer, wider and flatter - and propel themselves using an extra-long canoe paddle that's as tall as the rider. Apart from balance, the sport requires strength in the core, arms, back and shoulders. It's a great workout for the heart.

Together, Pilates and stand-up paddle boarding complement each other. Karen Mirlenbrink, a fitness trainer and stand-up paddle boarding racer from Florida, US, is widely regarded as the creator of SUPilates in 2011.

"For Pilates followers, stand-up paddle boarding is a great activity to highlight how various exercise movements benefit the body," says Clark, who is a veteran Pilates trainer who started stand-up paddle boarding a year ago.

"Stand-up paddle boarding is also a great activity for those new to Pilates because students will immediately feel the various muscles - that we try to activate in a studio - being turned on automatically because they are on a very unpredictable, changing surface.

"Similarly, while students in a Pilates class may think they have really good technique and find the movements easy, stand-up paddle boarding will highlight any flaws and movement issues - you'll fall off the board."

Fresh from having a baby four months ago, my goal for my virgin SUPilates outing was just that: not to fall off the board. My deep abdominal muscles have not yet returned to their pre-pregnancy state, which was honed through years of training for triathlons and trail running.

Just standing on the paddle board or doing the basic Pilates roll up (like a crunch, but with straight legs) is challenging.

Fortunately, I'd tried Pilates a couple of times, and stand-up paddle boarding once - on a choppy sea while on holiday in the Philippines last year during typhoon season. So I wasn't going in blind.

After a quick demonstration on land on how to paddle, Clark launched us into the water. We newbies knelt on the board for better balance and paddled out to sea, a relatively easy thing to do.

We gathered about 100 to 200 metres offshore, and Clark began the Pilates circuit.

Starting with the roll up, we then moved to progressively tougher moves, including the teaser (like a crunch, but with legs straight at a 45-degree angle), plank, quadruped (on hands and knees, then raising and extending alternate arm and leg), push up, side lift and spine twists.

Because the exercises are done seated or in a horizontal position, I found it quite easy to stay on the board, although it was harder to hold the position due to the added instability. The weaker the muscles, the more the board rocks.

It didn't help that, according to Clark, the conditions on this day were rougher than usual. We spent about half the time paddling out as we kept drifting back to shore.

A bit further out at sea was a group of about 10 people doing yoga on stand-up paddle boards - another trendy fusion fitness activity. With yoga's focus on meditation, I wondered how difficult it would be to keep calm amid the chop.

After about 20 minutes, I garnered enough courage to attempt standing up on the paddle board. The secret is to find the sweet spot between being too tense and too relaxed. Standing up took much more focus than kneeling, and immediately I could feel my core working. I paddled gingerly, careful not to overreach and lose my balance.

In the standing position, Clark had us do squats with paddle in hand and arms extended overhead. This was my downfall - literally. Just a slight loss of focus and the board flipped, sending me crashing into the sea.

"I like the fact that you're always engaged when doing SUPilates," says Demarco, a strategy director at a global law firm, after the 60-minute class.

"If you were doing Pilates exercises on a mat in a studio, you could stop and rest. But here, out at sea, you can't take a breather. You've got to be aware of your surroundings and you've got to keep paddling to make sure you don't crash into shore."

Because you've got to be constantly turned on, Clark says you burn more calories and the body is transformed more quickly with SUPilates compared to regular Pilates.

For me, the attraction of SUPilates lies in the setting: sun, sea, the sound of lapping waves, fresh air and a cool breeze. Give me that over the confines of an air-conditioned gym any day.

PilatesAthlete offers SUPilates classes at HK$400 per person for 60 minutes, including board hire. Participants need to be able to swim 50 metres. pilates-athlete.com