CRUISE SHIPS can be good for your health. They offer relaxing stress free travel, clean air (there is no pollen or air pollution at sea), healthy meals -- if you watch your diet -- and lots of special features to keep you fit.

The vessels present no problem should you require a special diet or if you just do not want to indulge yourself in the eating orgies cruise ships are known for. Virtually all ships--with prior notice before sailing--will provide you with just the diet your doctor ordered: diabetic, low-salt, low-fat, vegetarian and others.

Actually, eating habits afloat are changing. On the Queen Elizabeth 2, for example, smack in the middle of the menu and surrounded by seven-course, mind-boggling, stomach-stretching suggestions, are the "Golden Door" selections of the day: low-calorie, nutritiously balanced selections. And, according to veteran dining room personnel aboard, eating trends are definitely moving away from extra helpings and second desserts and toward lower calorie items and even people sharing dishes.

Capt. Peter Jackson, who was master of the QE2 until last January and will soon retire from Cunard Lines, attributes, at least in part, his decades of robust health at sea to sensible eating. "Just because we serve five meals a day doesn't mean you must partake of all," he says. "I eat sparingly even when entertaining guests at my table."

Another of the captain's suggestions for staying fit at sea: Don't smoke and stay away from people who do. Some ships have set aside part of their dining rooms for people who do not smoke. There also may be lounges for nonsmokers. Ask before you book a cruise and then request such facilities when you board the ship.

All cruise ships have physicians. The QE2 has two, one a surgeon. The liner also has several nurses, a physiotherapist and, on some cruises, a dentist. The QE2 has a complete operating room, X-ray equipment, a dental office and a well stocked pharmacy. The pharmacy is important, says Dr. Bill Davis, the ship's regular physician-surgeon, because so many passengers run out of medications that they must take. He advises passengers to pack ample supplies of medications when they go on a cruise.

Physicians and nurses on cruise ships are there not only for emergencies, Davis points out. They also will attend you if you need supervision for an existing medical problem--blood pressure determinations, blood and urine tests, for example. And the physiotherapist will continue treatments prescribed for you at home. There is a charge for all the services.

If you have a medical problem that may need attention aboard ship, it is advisable to notify the cruise line at the time of booking, preferably at least three weeks before sailing. Every effort will be made to accommodate you. Ships offer a surprisingly wide array of sophisticated supportative and recuperative facilities. For example:

* Kidney dialysis on some sailings of Holland America cruise ships from New York and Miami, aboard one of Delta Queen Steamboat Company's paddlewheelers on the Mississippi and aboard two sailings of Royal Viking Line--one from London to Athens with stops in between, the other from Athens to the Greek islands, Russia and Turkey.

Each of those sailings will include a physician specializing in kidney dialysis, several specialist nurses and all the equipment necessary for dialysis. Persons requiring dialysis generally require three, three-hour sessions a week on the machines to remove waste products from the blood that diseased kidneys cannot. Except for the hours on the machines, dialysis passengers are free to enjoy the ships and the ports of call.

For information: Unique Tours, 620 Landing Lane, Baiting Hollow, New York 11933. (516) 727-1182.

* Rehabilitation for post-coronary patients aboard the Norwegian American Cruises Sagafjord and Vistafjord from Fort Lauderdale to the Caribbean and also to the Mediterranean, and from San Francisco to Alaska. The program is headed by Dr. Leo Rubin, a cardiologist, and a staff that includes a psychotherapist, a nutritionist and other health specialists. It consists of a series of workshops dealing with subjects essential to the cardiac patient's and spouse's mental and physical well being. Topics include nutrition, stress, exercise, smoking, etc.

New Life, Cardiac Health and Fitness Services, 230 Park Ave., New York, N.Y. 10169. (212) 557-5540.

* Special facilities for passengers in wheelchairs and with other handicaps aboard the Sagafjord through the Panama Canal and aboard the Queen Elizabeth 2 to the Caribbean. Also available is a cruise of the Inside Passage to Alaska.

Flying Wheel Tours, 143 West Bridge St., Owatonna, Minn., 55060. (800) 533-0363.

Virtually all cruise ships have elevators, an important convenience for the elderly and for passengers with physical handicaps. Many stairways are somewhat steep. If an elevator is important to you, check the ship's plan carefully. On some ships, the elevator does not go to all decks. And get a cabin close to the elevator--otherwise you may be at one end of the ship, the elevator at the other.

If getting about is somewhat difficult for you, check at the time of booking on facilities for going ashore at ports you are interested in visiting. At some ports, ships drop anchor in the harbor. Then you must transfer from the ship to a tender and then from the tender to the pier. This takes some agility, especially when the seas are choppy.

Don't worry about seasickness. Ships have stabilizers, itineraries are carefully chosen to minimize rough weather and, when necessary, effective medication is available to prevent symptoms. Some veteran ship physicians prefer to give an injection of Phenergan, a potent anti-seasickness substance. Others dispense Transderm-V, a dime-size flexible disc with adhesive on one side. You paste the disc behind your ear and, for 48 hours, you absorb the right amount of scopolamine, another effective medication. Any medication may have side effects and contraindications, so be sure you are properly any anti-motion drug.