WHAT DO music lovers, physical fitness buffs, leisurely adventurers, nostalgia seekers and even unabashed swingers have in common?

"Theme" or special ocean cruises.

Cruises have been planned in the past either wholly, or in part, for cooks, weight-watchers, homesexuals, whale-watchers, singles, nudists -- and that's barely making waves among the varied voyages. An entire vessel may be booked by the like-minded group, or only a limited number of cabins may be set aside on a regular sailing.

Balmy breezes and movable feasts are no longer enough.

The shipboard programs often reflect the destinations. Sex, age, marital status, education, and income are all vital elements in cruising, and the lines feel they will continue to stay afloat on economically rough seas if they can provide enough fun and stimulation to convince more landlubbers to get their feet wet. One answer is to dream up a theme or design a cruise-with-a-purpose.

Here is a look at some of the special "incentive" cruises scheduled for the 1983 season -- in each case your travel agent can usually provide brochures, handle bookings, or make other suggestions:

"VANITY FAIR YEARS" WORLD CRUISE: Sailing east instead of west for the first time in seven years, Holland America's flagship, the S. S. Rotterdam, will leave New York Jan. 8 on its 1984 world cruise, which will visit 25 ports in 18 countries in 96 days.

Intended to be "far more than just a trip around the world," the cruise has been given a theme, "The Vanity Fair Years." Thus the 38,000-ton Rotterdam will recall with elegance and ambiance the 1914-1937 period with special events, parties, music and entertainment, including "unusual shore excursions to sophisticated places of the period like Raffles in Singapore and the Jockey Club in Buenos Aires."

The cruise is divided into four segments for those who cannot take the entire journey: New York to Cape Town, South Africa, (28 days), Cape Town to Hong Kong (31), Hong Kong to Los Angeles (21), and Transcanal-L.A. to Fort Lauderdale (16). Cost for the full cruise ranges from $13,955 to $45,455 per person double occupancy (the last Holland America world cruise embarked fully booked leaving a waiting list).

For information, Holland America Cruises, 2 Penn Plaza, New York, N.Y. 10121 (800) 221-6842.

MUSIC FESTIVALS AT SEA: One of the most popular types of special sailings in recent years, classical music cruises have been combining first-class performances by major artists with a unique opportunity to meet the musicians and singers informally and socially, attend rehearsals, enjoy good food and visit interesting ports of call.

Paquet French Cruises is a pioneer in the field, having offered 23 of these melodic sessions on the high seas since 1968 aboard the Renaissance and the Mermoz. The line usually schedules two festivals annually, one in the Caribbean in January and one in the Mediterranean in the fall, and shore concerts are included. This year artists such as Byron Janis, Ruggiero Ricci, Shlomo Mintz, James Galway, Maurice Andre and the Chamber Orchestra of Poland will perform aboard the Mermoz, which sails from Toulon, France, on Sept. 1 for a 14-day cruise to Malta, Greece, Turkey and Italy. Prices range from $2,680 to $6,580 per person double occupancy.

For information, Pacquet French Cruises, 1370 Avenue of the Americas, New York, N.Y. 10019 (800-221-2490).

Royal Viking Line will feature two Mediterranean/Adriatic festival cruises aboard its highly rated Royal Viking Sky embarking from Piraeus, Greece, the port for Athens, on April 25 and May 9:

Guest artists will include Jorge Bolet, Victoria de los Angeles, John Browning, Montserrat Caballe, Anna Moffo and Ruggiero Ricci (not all appearing on the same cruise). Ports of call are Catania and Livorno, Italy, Villefranche, France, Kotor and Dubrovnik, Yugoslavia and Venice. Shore concerts will also be offered. Rates run from $2,156 to $8,190 per person.

For information, Royal Viking Line, One Embarcadero Center, San Francisco, Calif. 94111 (415-398-8000).

"SWINGING ON THE RHAPSODY": This a nonmusical kind of cruising that answers to a different drummer. The title refers to a he-donistic Caribbean vacation scheduled by the North American Swing Club Association, described as a group of national and international swing clubs and publications, and by Club WideWorld, which a spokesman said sponsors various lifestyle tours that are "sexually/socially oriented."

From April 30 to May 7, "adventuresome couples" will embark from Miami on the Paquet French Cruises 25,000-ton S.S. Rhapsody for ports in Jamaica, Grand Cayman and Mexico. The Brochure promises "monokini sunbathing in an area set aside just for us" (the swingers expect to fill 20 cabins -- assuming double occupancy, that would be limited to about 40 passengers on a vessel with a full capacity of 850).

The brochure piques interest without being too explicit: "A new island appears on the horizon... a new couple... -- both tempting exploration." Its responsibility clause is somewhat unusual but understand-able, and swingers will get the message: Wide World states it won't accept any blame for any claims "arising out of... loss of enjoyment, upset, disappointment, distress or frustration, whether physical, emotional or mental, resulting from the act or omission" of anyone other than the tour operators.

For information or reservations, contact the booking agent, Paradise Travel, 68733 Perez Rd., Ste. 6, Cathedral City, Calif. 92234, or Club Wideworld, P.O. Box 5366, Buena Park, Calif. 90622.

THEATRE AT SEA '83: The Theatre Guild Inc., promising a different kind of excitement on the same Paquet ship -- but not on the same cruise -- is accepting reservations for its fifth production afloat. The previous programs were on ships of other lines. Pacquet is delighted to have the group, which is expected to fill at least one-third of the Rhapsody's staterooms.

Sailing from Miami on May 14, the Rhapsody will visit Jamaica, Colombia, transit the Panama Canal, and stop at Acapulco, Mexico. Before the theater fans reach Los Angeles on May 28, they will have been entertained by Anne Jackson, Eli Wallach, Coral Browne, Vincent Price, Maureen Stapleton, Richard Kiley, Patrice Munsel and Dick Shawn.

There will be "nine gala evenings" of comedy, tragedy and drama featuring scenes from Shakepeare to Noel Coward, plus cabaret entertainment, discussion groups, lectures and seminars. Special events are planned in each port "exclusively for Theatre At Sea adventurers."

Rates for the 14-day journey, including round-trip air from passenger's home town, range from $2,345 to $4,455 per person, based on double occupancy.

Free brochure from Theatre Guild Abroad, 226 West 47th St., New York, N.Y. 10036; information and reservations, Godfried Bruer (212) 265-6170.

GETTING "SHIPSHAPE": Holland America had scheduled a "Cruise to Fitness" on the Volendam to introduce passengers to the "German spa concept," with supervised diets, seminars and on-board weight reduction/maintenance activities. The program was canceled due to insufficient interest. But there's still time to get "Shipshape at Sea."

That's the name of Sun Lines' Cruises program of health and fitness, which has been offered aboard the Stella Solaris on a series of trans-atlantic and Caribbean cruises. It will be resumed on sailings after Nov. 22. Classes and lectures on diet management, exercise and stress reduction are open to all passengers at no extra charge.

Information, Sun Line Cruises, 1 Rockefeller Plaza, New York, N.Y. 10020 (800-223-5567).

A number of cruise lines, aware of the growing emphasis on fitness in this country, provide facilities such as gyms, massage rooms (usually an extra fee) or saunas. Swimming pools are a standard fixture. Passengers can job on deck.

Cunard's "Golden Door" fitness program on the QE2, with instructors from the California spa, will develop a personal plan for each passenger without charge (a week at the Golden Door normally costs $2,000, Cunard notes). This includes daily fitness activities (16 classes to choose from), lectures and low-calorie/high-energy menu selection. Cost of a transatlantic cruise (intermediate season) on the QE2 runs from $1,360 to $5,865, and includes return flight from London to Washington on British Airways.

Information, Cunard Line, 555 Fifth Ave., New York, N.Y. 10017 (212-880-7500).

Myriad adventure tours include cruises emphasizing the environment, such as salen Lindblad's expeditions to Alaska, Asia, the Indian Ocean and the Amazon, and the new Arctic air/sea packages by Greenland Cruises Inc. There are also trips that emphasize cultural aspects of the destination, like Swan Hellenic Cruises and the China voyages of Pearl Cruises of Scandinavia. Passengers attend lectures and receive other expert guidance designed to broaden their knowledge and appreciation of the areas.

It is a mistake to assume from every brochure extolling the wonders of a special interest cruise that a majority of the vessel's staterooms have been automatically reserved for members of that group (which naturally would almost guarantee a distinctive flavor on the voyage, as in the case of the adventure cruises described above). Nor should you assume that the line itself is actively promoting the specific program described.

Brochures (shells) containing blank areas for personalized printing are often supplied by a cruise line to tour organizers upon request if they create a package and intend to book space for a group. The tour operator can then add his own message to sell the cruise. Perhaps as few as 10 passengers out of 1,000 are required to sign up in order to qualify for group terms. As always, study all printed materials, understand who is operating the trip, read the "responsibility" and other disclaimer clauses, and request further information if needed from your own agent or the carrier.

If you sign up for a cruise without any idea that a special interest group is aboard, and then suddenly learn about the activity as you head out to sea and wish you could be part of the festivities -- ask the purser. Sometimes you can participate by paying an additional fee -- $250 in the case of the Theatre Guild's presentation, for example.

"Love Boat" cruises are in a special category. The ABC television series has given the cruise industry millions of dollars of free publicity and whetted the appetites of many viewers for a breath of salt air.Portions of some of the earlier programs were filmed in the Caribbean aboard Princess Cruises' Pacific Princess. Some passengers (not every cruise buff wanted to be bothered) who knew which sailings were selected had an opportunity to be "extras."

Last May the producers shot segments for three two-hour specials aboard the Sun Line's Stella Solaris in the Mediterranean. The "Love Boat" cast will sail next on the Pearl of Scandinavia's 14-day "China Explorers" cruise departing Hong Kong on May 21. Due to cabin needs of the cast and other show personnel, regular passenger capacity of 450 will be reduced to 250 -- the ship was reported almost fully booked last week. The line notes that cancellations do occur, and they will keep a wait list. Rates, including air fare from the West Coast and "all non-optional shore excursions" in the People's Republic of China, are $3,370 to $7,500 per person, double occupancy.

Information, Pearl Cruises, Pier 27, San Francisco, Calif. 94111 (800) 227-5666.