Scrapbooking, Elvis, tulips, Harleys: At this point, there’s no passion you can think of that hasn’t inspired a cruise of some sort, so don’t even try.

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,” notes 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” — that hasn’t been thought of before, WHICH YOU CAN’T — “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,” observes 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 the cruise lines and private charters, so many that whole Web sites are now devoted to sorting out the offerings, such as ThemeCruiseFinder.com.

“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?” Right now, he has more than 500 upcoming cruises listed on his site. Some of these Moses sells himself (such as, say, a 12-night photography cruise through the Mediterranean in June), although he insists that his site will list any theme cruise for free, regardless of its backer.

Lean to the left — or right — at sea

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 64-year-old 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 onboard with her, it said.

“These are my rock stars,” says Dewenter, who soon 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!’ They couldn’t understand what it was about.”

But for Dewenter, the National Review cruise meant the chance to interact with people whom she’d always 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, however briefly. “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.”

Evelyn Hess found her own floating haven last February (this one on the opposite end of the political spectrum) when she took a Caribbean cruise sponsored by Air America radio with her elderly father. A young mother from California’s Bay Area, she, too, sought the consolation of like-minded sailors. Rachel Maddow was there; so were a pre-Nobel Prize Paul Krugman and liberal radio personality Jim Hightower. And last fall, Hess took another cruise for liberals, this one sponsored by the Nation magazine. That one taught her that a shared politics does not guarantee smooth sailing.

“There were no sacred cows. They even yelled at (Nation editor) Katrina vanden Heuvel,” laughs Hess, describing some of the delicious seminars in which she took part. The Nation is run out of a Manhattan office building, so there were lots of New Yorkers on board, “and they had an interesting edge to them,” she reports. “Panelists would get argumentative and the crowd would take sides, and there would be yelling. It’s lots of fun … and really liberating because while we might not all agree on Jesse Jackson” — who has been known to sail on a Nation cruise himself — “we’re all (angry) at the same stuff.”

Feeling liberated

It has been almost 20 years since Nancy Tiemann first set foot on a nude beach. “I always thought, ‘Those people are surely up to something,’” says the former banker who then, as now, is happily married to a former lawyer. The Austin, Texas, couple’s shared former-ness can be traced to a booming cruise charter business they founded in 1990, Bare Necessities.

“We were probably one of the original theme cruisers,” says Tiemann, who started small, booking her first nude charter on a small dive boat. “From those humble beginnings,” she laughs, “we’ve grown exponentially as the popularity of social nude vacationing has grown.”

For obvious reasons, Bare Necessities can book only full-ship charters, but that doesn’t mean its options are limited. Next month, more than 2,000 of Tiemann’s clients will set sail aboard the Carnival Legend for a seven-night cruise out of Tampa, Fla., sans clothing. Once they’re safely away from port, that is.

“In the formal dining room, we’re clothed” — although not necessarily formally, Tiemann notes — “and also when we get out of international waters and approach a country where nudity is illegal, which is most of them.”

For Tiemann, theme cruising is about the freedom to be oneself without fear of retribution. “It’s an atmosphere where you start off with something in common. It’s jazz or it’s wine tasting or it’s being gay and proud, or being a nude vacationer and proud.”

The itineraries are as various as singles cruises to “meeting people’s medical needs so they can go cruising,” says ThemeCruiseFinder’s Moses, who points out the growing number of niche charter groups, among them Dialysis at Sea, Buddy Cruise (for Down syndrome families) and Autism on the Seas.

Where runners sail

Danny Roberson is a serious runner, as in marathons, half-marathons and the like. “I really wasn’t a cruise person per se,” says the 47-year-old banker, who lives in Winston-Salem, N.C. “I think I had a normal apprehension that folks have, that I’ll be stuck on the boat, that the people won’t be like me, that they’ll all be old or they’ll all be teenagers on spring break.”

In 2003, he heard about a cruise up the Eastern seaboard that he couldn’t resist, however, one that would allow him to run practically anytime, anywhere. Sponsored by Runner’s World magazine and calling at various ports in Canada and New England, Roberson’s cruise offered the chance to run with the best. Frank Shorter, the 1972 gold medalist in the Olympic marathon, was on board, “and when you have Frank Shorter on your cruise, the locals meet the boat.” When the ship docked in Charlottetown on Prince Edward Island, it was greeted by upward of 100 area runners, who gladly introduced the cruisers to trails that most tourists never see.

“When we stopped in Bar Harbor in Maine, Joanie Benoit Samuelson met us. She had come down to run with us in Acadia National Park,” Roberson remembers, admitting that he could keep up with the legendary marathoner only during warm-ups. Once they were back on the ship, the running continued in the ship’s gym and in endless ovals around the deck.

“After a while, you start to feel a real bond with some of these people,” Roberson says. In the years since, he has sailed on two more running-themed cruises, both to Alaska. It’s exhausting, it’s all-encompassing, and “to me, it’s the only way to cruise,” he reports.

Blues on the deep blue

“It’s an immersion experience unlike anything else that’s possible,” says Ted Newland, 59, of Fairfax, Va., whose passion is not running but music. In fact, he’s set to embark on his fourth Bluescruise, a full-ship Holland America charter that sets sail from Fort Lauderdale, Fla., every year at this time. At most blues festivals (the kind that cater to landlubbers), he notes, “you go, the musicians come, and everyone leaves at the end of the day.” On a cruise, “everyone is together all the time.”

Newland has loved the blues for most of his life, but a cruise brings both “isolation and integration,” as he puts it. “The world disappears — really.” That’s not to say you won’t still find yourself doing typical cruise-ship things, like standing in the buffet line, but it’ll be with blues star Taj Mahal and you’ll be chatting the whole time.

Musicians and cruisers mingle happily everywhere and, as a bonus, “you get to learn their personalities,” Newland says. “There are a few, not many, prima donnas … but most are incredibly genuine human beings. When you’re among people for a week, it’s hard to completely hide what you are.”

“It’s the totality of it” that makes a Bluescruise so special, he concludes. And that absorption changes not just how they see themselves, but how they see cruising, too.

“OK, let’s be honest here,” Newland says. “It is not inaccurate to describe the age of a typical Holland America patron as nearly dead. On the Bluescruise, it boggles the mind to contemplate that you are on the same cruise line.”