There's hardly a hobby or interest that hasn't already been or that won't someday be the subject of a theme cruise.
People who love to sew will be kept in stitches, for example, on a seven-day sewing cruise aboard the Costa Romantica.
Other theme cruises let passengers pursue such pastimes as square dancing and quilting, indulge obsessions for chocolate and wine-tasting, or spend days solving crossword puzzles and shipboard murder cases.
The reason for the popularity of themes at sea is that, like magnets, they attract people with similar interests. Their tight focus creates an even more intimate and convivial onboard atmosphere than usual, and people who might not otherwise have travel companions gain an instant set of friends when they board.
While themes frequently are put together by cruise lines themselves, a recent trend is for outside tour operators to sponsor smaller groups organized around specific themes.
Here are some on the drawing board:
A Clearwater, Fla., cruise-tour operator plans the following four cruises in January. Rates for each are quoted without airfare (Call  677-9412):
* A quilting cruise to Mexico aboard Costa Romantica (Jan. 16-23) will be led by Doreen Speckmann, well-known in the quilting world. She and three other teachers will conduct daily 3- to 4-hour quilting sessions. Participants range from novices to expert members of the Quilting Guild. The cruise departs from Miami. Rates from $994 a person.
* Hands-on sewing classes will be given on Costa Romantica's seven-day Creative Machine Cruise departing Miami Jan. 30. Robbie Fanning and five other teachers will focus on an individual specialty -- cloth doll pins, machine quilting, and serger techniques. Fares from $994 a person.
* Instead of just taking snapshots on your next cruise, why not paint pictures? A seven-day Art Escape to the Caribbean departs Jan. 15 from Fort Lauderdale on Celebrity's Zenith. The cruise has five watercolor teachers aboard. Ports include Montego Bay, Grand Cayman, Cozumel, Playa del Carmen and Key West. Fares from $992 a person.
* Another cruise for art's sake recruits watercolorists Tony van Hasselt and Judi Wagner, who will conduct six morning classes aboard the Crown Jewel's Jan. 22-29 Caribbean cruise. Lessons carry over in port stops as passengers learn to paint alfresco with minimum equipment. Fares from $994 per person.
Other theme cruises include:
* Big Bands at Sea, from Nov. 27-Dec. 4 on the Norway. Among the performers scheduled are Ray Anthony and his orchestra; the Jimmy Dorsey Orchestra under the direction of Jimmy Miller with featured vocalist Nancy Knorr; and Les Elgard and his orchestra. Fares from $1,299 a person. Call (800) 327-7030.
* Learn the Texas two-step in a tropical setting: Cunard Countess sails to the Caribbean Dec. 4 for an American Country Music Festival At Sea. The one-week cruise features country music and square-dance lessons, karaoke sing-alongs, and TTC country cooking demos. A half-dozen more such festivals are planned for '94. Fares from $1,195 a person, including round-trip airfare to San Juan. Call (800) 221-4770.
* If you've ever wondered whether royalty eats better than you do, you'll have a chance to find out on the first segment of the QE2's 1994 world cruise. The 13-day sailing, from New York to Los Angeles via the Panama Canal, brings aboard several members of the Club des Chefs des Chefs (the official chefs to royalty and heads of state), who will prepare dishes and lecture on the art and tradition of gastronomy.
Also on this segment will be the Philharmonia Virtuosi Chamber Orchestra performing masterworks from the royal courts, and Victor Borge will entertain. Fares begin at $3,360 a person, double, and include round-trip economy air from 79 North American cities. Call: (800) 221-4770.
* The good news for Civil War buffs is the Delta Queen Steamboat Co. has come through the floods on the upper Mississippi in fine shape; and, according to spokeswoman Patti Young, their steamboats are now safely cruising the calm waters of the Ohio, Cumberland, Tennessee and lower Mississippi rivers.
However, they're rerouting some cruises even as we speak, so details are still unconfirmed.
At this writing, its two seven-night Civil War theme cruises on the Mississippi Queen still will depart on Aug. 18 (from Memphis to Chattanooga) and again on Aug. 25 (from Chattanooga to
Cincinnati), but the enrichment program is not yet in place. In the past, Pulitzer Prize-winning authors have been aboard to explain the war's history.
Both cruises include tours of Civil War forts, plantations, battlefields and cemeteries and a stop at Pittsburg Landing. Call: (800) 543-1949.