Cruise-ship travel generally brings to mind hours of sitting and endless all-meals-included eating, rather than burning calories. Yet on a 14-day sail through the Panama Canal aboard Celebrity’s Summit recently, many passengers kept up their usual exercise routines – or even kicked them up a notch.

At the 25,000-square-foot AquaSpa, home of Summit’s fitness center, scents of frangipani, ginger, jasmine, and coconut emanate from nearby treatment rooms. The gym, equipped with a Pilates Body Reformer, free weights, benches, exercise bikes, a rowing ergometer, free climbers, weight machines, fitness balls, resistance bands, yoga mats, and an aerobic area, features floor-to-ceiling windows and panoramic sea views. The fitness center has a total of 19 treadmills.

For the better part of a decade, gyms have become standard onboard cruise ships, which constantly strive to round out their offerings and keep pace with their land- and sea-based competitors.

“Over the past few years, we’ve seen a significant increase in the number of guests who want to start or continue a fitness regimen or enjoy some personal pampering while at sea, which is why we’ve gone to great lengths to equip our spas with the latest technology and treatments,” said the president of Carnival Cruise Lines, Bob Dickinson.

The luxe Crystal Cruises teaches Tai Chi through the Tai Chi Institute on selected sailings and has health-and-fitness-themed cruises.

Celebrity offered its first fitness classes and personal training sessions in 1997. As of 2003, in addition to step aerobics and other standard offerings, passengers could, for a nominal fee, sign up for Pilates, spinning, yoga, and other specialty classes.

A spokesperson for Celebrity said that 40% to 45% of guests use Summit’s fitness center. On the voyage through the Panama Canal, the class sign-up sheets often had wait lists.

Like its land-based counterparts, Summit’s gym had an ebb and flow of visitors. “Mornings are busiest – 6:30 a.m. to lunchtime,” said Cherie Mihaere, who taught classes on the cruise and offered personal training.

Alison Lockyer, the fitness director, said, “It gets busy then at 3 p.m. until dinner.” Days “at sea,” when the ship does not call at a port, see the most traffic, she added. Most passengers flock to the treadmills and cross-trainers, she said. Some passengers had the unique experience of working out alongside the resident performers from Cirque du Soleil, whose workouts made everyone else’s look like a stroll on the deck.

Stephanie Crown, 28, of Austin, Texas, who had recently given birth to her first child but looked ready to run a marathon, took an “abs and bum” class with her husband at Summit’s gym during the Panama Canal cruise. It was her first visit to the ship’s fitness center, but her husband had been four times.

“I haven’t been doing as much as he has,” she said. However, she explained that they both had resolved to take the stairs rather than use the elevator aboard the ship.

“It’s a good way to burn a couple of calories,” Ms. Crown said. “Unless, of course, I’m in an evening dress, then I’m doing the elevator. With the stairs you definitely feel it after a couple of flights.”

They occupied a stateroom on the sixth level of the ship, and the uppermost deck towered six flights above. “So when we go up to the top floor,” she said, “we race each other. That’s about as good a workout as we just did,” she said, comparing it to the vigorous strengthening class.

Another devotee of walking the ship’s stairs, a trim Londoner named Una O’Halloran, on her eighth cruise, explained her fitness strategy.

“We never use the lift at all – even on formal nights,” she said

She also popped into the gym periodically, while her husband swam in the pool. “Ten minutes on the bike – just enough to make you feel good,” she said.

Fran Crampton, of Fort Pierce, Fla., rounded Sunrise Deck’s track most days of the cruise on an early-morning walk. A veteran of 12 cruises, Ms. Crampton, 66, sported a Curves T-shirt declaring, “There’s no excuse not to exercise on a cruise ship.” At home in Florida, she maintains a three- to four-mile per-hour pace during her daily 60-minute jaunt, a speed that did not abate at sea.

Ms. Crampton also occasionally visited the ship’s gym. “I did the treadmill one morning, and my husband did the bike,” she said, adding, “If you’re going to eat, you’ve got to exercise.”