For the guy sitting next to me, a fit triathlete in his 50s, it was all about the bike. For me, it was all about making something I detest — spin class — something resembling fun. I looked around at those gathered in the infinity edge Garden Pool on MSC Divina as we cruised the Bahamas, and saw that most of the passengers ready to take the first aqua cycling class at sea — on Aquabikes by Hydrorider — were more my speed: fitness wannabes rather than spin addicts.

Maybe that's because this didn't feel like a spin class. Yes, we were seated on bikes, but in pool water up to our waists, and facing the ship's gently churning wake with nothing but blue horizon all around us. "I got this," I said. Mr. Triathlete smiled. "That's the right attitude," he replied.

And then we met Giancarlo, a lanky, mid-40ish former member of an Italian cycling team, who was pacing the edge of the pool, psyching us up for the next 30 minutes of what he promised would be the most “fantastico” workout ever. He cued the music, hopped on his bike, and we were off.

With our wheels whizzing sub-aquatically as we found the rhythm of the music, waves of water drenched us with splashes that were so much better than the unholy cascade of spin-class sweat. Despite the lighthearted ambiance, our cycle pro pushed us hard: Stand up, sit down, faster, slower! This low-impact cardio workout had my heart pumping, and I felt the burn.

After a while we cooled down, dismounted, and climbed from the pool, laughing. A mother and daughter I’d gotten to know on board greeted us. “You did great,” the thirtysomething daughter said. “Didn’t she, Mom?” Mom didn’t answer right away — she was busy checking out Mr. Triathlete. Finally, a reply: “I wasn’t really watching her.”