When fans sway to the music at a Cayamo concert, it’s not always because of a catchy beat.

Seats actually rock and roll on the Norwegian Pearl cruise ship as Grammy Award winners like John Prine and Steve Earle perform for passionate crowds at the annual music festival in the Caribbean. The stage can also shift along with the ocean waves.

“My legs are doing fine, but my voice is just getting its sea legs,’’ Prine told his opening-night audience after hitting a wobbly note.

Prine, Earle, and John Hiatt were the headliners at this year’s ninth Cayamo cruise, a seven-day celebration of singer-songwriters that took place the first week in February. But the genre-spanning festival, which features folk, blues, and bluegrass as well as country, rock, and gospel, gives equal attention to talented but lesser-known artists with dedicated followings. This year, that group included John Fullbright, Shawn Mullins, David Bromberg, Shawn Colvin, Lucinda Williams, Buddy Miller, Chris Stapleton, Kacey Musgraves, and three husband-and-wife pairings: Larry Campbell and Teresa Williams; Jason Isbell and Amanda Shires; and Abner Ramirez and Amanda Sudano, a duo puckishly known as Johnnyswim.

Sixthman, the Atlanta-based creator of Cayamo, has also tailored musical cruises for fans of rock, country, blues, Elvis, Lynyrd Skynyrd, KISS, and Kid Rock. However, no cruise has a more devoted fanbase than Cayamo, most of whom are aging baby boomers who remember Woodstock more clearly than their Internet passwords.

Of the 2,000 passengers on this year’s voyage, more than half had been on at least two previous Cayamo cruises and 101 people had been on every trip since the first one in 2008. I sat next to one of those nine-timers, as they’re known in Cayamo circles, at Prine’s opening-night concert in the 920-seat Stardust Theater, the largest venue on the ship.

“It’s like a big family here,’’ said Sharon Meyer, a retired state government worker from Boulder, Colo. “It’s not like a regular cruise. People come mainly for the music, not just to eat and lie in the sun.’’

Many of the musicians are also Cayamo veterans.

Seventeen acts in this year’s lineup had performed on at least two other Cayamo cruises. Mullins, an Atlanta native best known for his 1998 hit “Lullaby,’’ is the reigning champ, having played on all nine. But Miller isn’t far behind. A producer and performer who has worked with Johnny Cash, Levon Helm, Emmylou Harris, and Willie Nelson, Miller has missed only one Cayamo outing — and that was because he had a heart attack a week before the 2011 cruise that led to triple-bypass surgery.

“Some people thought that was a lousy excuse,’’ he said with a laugh one night in the ship’s casino.

For musicians who spend more time on the road than long-haul truck drivers, Cayamo offers a relaxing week of camaraderie and collaboration with fellow performers they rarely see the rest of the year. Guest appearances at Cayamo concerts are as common as seasickness jokes.

“It’s a chance to see a lot of friends and people you admire,’’ said Campbell, a multi-instrumentalist who spent eight years touring with Bob Dylan’s band. “We’re musicians, but we’re also fans.’’

My wife and I signed up for Cayamo at the urging of longtime friends, a Miami couple who share similar musical tastes. We never had any desire to go on a cruise until our friends, who are six-time Cayamo cruisers, convinced us that we would be spending most of our time listening to music instead of pigging out at the buffet tables, drinking funny-sounding cocktails, or slathering ourselves with suntan lotion.

They were right. You can attend concerts practically around-the-clock, starting most days around noon and stretching into the wee hours of the morning. We were usually too exhausted to stay up for the late-night shows, though I made it to a couple of midnight jam sessions where it seemed half the musicians on board showed up to play impromptu gigs as if they were hanging out in your living room.

Music gorging wasn’t the only activity on the ship. You could also bowl, swim, play basketball, gamble, or take tai chi and sand art classes. My wife did tai chi every morning because she thought the instructor was cute, while my extracurricular schedule consisted mostly of losing money at the poker tables.

We stayed in a comfortable stateroom with a queen-size bed, a desk, a bathroom, a small sitting area, and a sliding glass door that led to a private balcony, which we hardly ever used. Prices range from $1,315 per person for an interior room to $5,775 for the ultra-luxurious owner’s suite, which includes butler service for those too lazy to fetch their own food and drinks.

The prices included everything except soda, liquor, transportation to the departure port in Miami, excursion fees during stops in St. Maarten and Tortola, and a daily gratuity charge that ranged from $12.95 to $14.95 per person based on your type of room. We did brief tours of both islands, but many passengers preferred to stay on the boat and enjoy a quiet afternoon resting their ears for the next concert.

We heard few complaints on our cruise, and most of them were minor gripes about things like the occasionally fickle sound system and the thickness of the toilet paper.

“The toilet paper is so thin you couldn’t wipe a smile off your face,’’ Hiatt quipped.