The Dirty Dozen Brass Band performing on deck in January on the Jam Cruise. Robb Cohen

Les Claypool, the bass-guitar player and leader of the oddball psychedelic band Primus, has a taste for the strange. He has, after all, written a song called "Pork Soda" and has named another of his groups Colonel Claypool's Bucket of Bernie Brains.

So it comes as little surprise that he got a charge from the surreal vision he witnessed this last January onboard for Jam Cruise 2, a four-night sea outing for fans of improvisational rock bands.

"Here we are playing at 4 in the morning, we're doing this demented, twisted version of "Another Brick in the Wall,' he said." You know, people are jamming and having a good old time and we're cruising past Cuba."

The experience proved so enjoyable for Mr. Claypool that he has signed up for Jam Cruise 3 next January. So did nearly 500 fans who registered to pay between $550 and $1,125 for next year's trip even before the musical lineup was announced. The cruise, which was organized by Cloud 9 Adventures in Boca Raton, Fla., is scheduled with multiple sets from jam band favorites like Galactic, Jon Fishman of Phish, North Mississippi Allstars, Ozomatli and DJ Logic.

The jam bands, however, do not have the seas to themselves. Also next January, the Legendary Rhythm & Blues Cruise sets sail with Taj Mahal, Dr. John, Susan Tedeschi and the Derek Trucks Band. It sold out a year in advance.

Although the concert industry does not track sales from music-related cruises, their number appears to be increasing, said Ray Waddell, who reports on the tour business for Billboard. The cruise business has plenty of room to grow with jam bands, said Roger Naber, one of the producers of Jam Cruise and a longtime producer of the Legendary Rhythm & Blues Cruise. "It's a new concept for this genre of music. They've only thought of cruises being for newlyweds or nearly deads. It is changing." These cruises, organizers said, are drawing customers who are slightly older than the usual rock festival crowds, who may be burnt out on the dirt, junk food and porta potties that come with most rock festivals.

"Now that I'm getting a little bit older, I'm not all about fighting the crowds, camping out in some big muddy field and stuff like that," said Marc Smith, 33, of Charlottesville, Va., who went on last January's Jam Cruise to Nassau. "With the Jam Cruise, you get your own cabin, a clean bathroom, food to eat, showers. All that stuff's important to me."

Music-theme cruises ‹ primarily jazz and big band ‹ have been popular for at least 20 years, said Gary Bongiovanni, the editor of the concert trade publication Pollstar. Not surprisingly, the music on those trips was geared toward a slightly older crowd, since older customers had more money to afford cruises. Today any number of travel agencies promote theme trips like oldies cruises, country cruises, even banjo cruises.

But charter music trips like the Jam Cruise differ from the more traditional theme cruises because they trade elements like black-tie dinners and bingo for a music schedule that can run around the clock.

This new circle of cruisegoers wants to sail with musicians who are willing to toss out standard set lists and jam as the muse calls them, often playing in spontaneous groups. While the audiences for these outings are not teenagers, they are not the A.A.R.P. crowd, either. Jam Cruisers, as participants call themselves, average between 25 and 35, and several music cruises attract crowds in their 30's and 40's, organizers said.

"The idea is definitely appealing to a younger generation," said Todd Wickersty, 33, of also of Charlottesville, a town known for its fondness of jam bands, who went in March for a weeklong cruise on which the jam band called moe played six nights. "I can't say that I would have enjoyed the nightime entertainment on the ship as much if moe hadn't have been there."

Beyond the enticement of shows and sun, the cruises also offer fans the chance to spend time hanging out with the musicians onboard. "The artists are accessible and available, and eat dinner with you and go snorkeling with you on the shore excursions," said Dale McGinnis, 34, of Melbourne, Fla., who was on the first of the two Jam Cruises last January and is planning to be there next year as well. "We're all on the same boat, literally and figuratively," he said.

For many younger cruisers, the highlights are hidden in unannounced or unanticipated music combos. Mr. McGinnis recalled one spontaneous jazz jam in an unused bar on a Jam Cruise that featured the bassist Rob Wasserman (who has played with the likes of Lou Reed and Elvis Costello), members of the Dirty Dozen Brass Band and Galactic, plus a fan onboard who just happened to play.

This kind of improvisational group falls together easily among blues, jazz and jam musicians. Whether the cruise environment will translate so easily to other smaller musical niches is unclear. But Josh Moore, the Jam Cruise band booker, said he would like to assemble alternative country and neo-soul cruises as well.

"I would imagine it would work with any situation," Mr. Claypool said, even for a heavy metal marathon like Ozzfest. "I'm not sure I'd want to be on it," he add. "But you could do it."

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