Scrapbooking, Elvis, tulips, Harleys: At this point, there's no passion you can think of that hasn't inspired a cruise, so don't even try. The phenomenon, "theme cruising," has intrigued scholars for centuries, which is to say since before cruising even existed. And few have spoken more eloquently on the subject than Alexis de Tocqueville.

"Americans of all ages, all conditions and all dispositions constantly form associations . . . religious, moral, serious, futile, general or restricted, enormous or diminutive," wrote the 19th-century Frenchman who, sadly, never lived to see the NASCAR cruise or the Star Trek fans cruise, but whose writings somehow prefigured both.

"As soon as several inhabitants of the United States have taken up an opinion or a feeling which they wish to promote in the world, they look out for mutual assistance; and as soon as they have found one another out, they combine."

And then, just when you think Tocqueville will explain why these combinings will one day find their way onto mega-ships in the Caribbean, the author of Democracy in America falls silent, to the everlasting dismay of cruise historians everywhere. Is it the fellowship? The social validation? The feeling of merging with a larger nautical whole?

The answers, as I discovered, are yes, yes, and definitely yes.

But first, a definition. Though the term is of recent vintage, theme cruising has been around as long as cruising itself. Sometimes it's centered on a hobby shared by as few as eight to 10 cabins, at other times entire shiploads. But, at all times, the participants are die-hard enthusiasts whose onboard activities must be customized accordingly.

"In the early days, we had lots of senior-citizen-type groups, which we still have," says Joan Levicoff of Carnival Cruises, "and lots of fraternal organizations, which we still have." At present, however, "if you can think of something that's going to be attractive to the cruising audience, then you have a potential theme on your hands."

And so, an explosion of sorts has occurred, although a controlled one.

"Fifteen percent of the sailings have a theme element, either the entire cruise or audiences within a ship," says Bob Sharak of Cruise Lines International Association, an industry group. That percentage has remained relatively constant over time, he says, "although the themes themselves have certainly evolved."

That's putting it mildly. At any given time, there are hundreds of theme sailings available through both cruise lines and private charters, so many that Web sites such as ThemeCruiseFinder.com are devoted to sorting out the offerings.

"People make fun of this, but the truth is, everybody's interested in something," says Howard Moses, who founded the site last year. "So why wouldn't you want to be with people who share that interest?"

Moses has more than 500 cruises listed on his site. Some of these Moses sells himself (such as a seven-night Air America Radio cruise to the Mexican Riviera in March with speakers including Robert F. Kennedy Jr., Howard Dean, and Ron Reagan), although he insists that his site will list any theme cruise for free, regardless of its backer.

It's not that Beth Dewenter didn't love traveling. It was cruising that she hated.

"I'm not a sun-and-sand person, so it had zero appeal," says the Dallas-area resident and retired teacher. But then, while flipping through National Review a few years back, she came across an ad for a Caribbean sailing on Holland America that the conservative publication was sponsoring. Pundits such as Bernard Lewis and Rich Lowry would be aboard, it said.

"These are my rock stars," says Dewenter, 65, who booked passage on the 2004 sailing over the objections of her friends.

"When I told people, they'd say, 'You're going on seminars? But it's a cruise!' " she remembers. "They couldn't understand what it was about. I mean, have you ever seen an episode of The Love Boat where people just talk?"

But for Dewenter, the National Review cruise meant the chance to interact with people she'd admired but knew only as bylines in a magazine, and to do it in a comparatively safe environment.

"It was meeting people from all over the country who in many ways were feeling constrained," she says. For her, the ship became, in effect, a conservative haven. "To be able to just sit down at a table and have a conversation without having to go through this magic dance of ferreting out political persuasions . . . it was great."