IF YOU'VE never sailed before, you probably picture a blissful week of doing absolutely nothing but relaxing in a deck chair with the waves to rock you, the sun to toast you and a steward to serve up cold drinks.

But cruises are as varied as the people who take them, and you might consider searching out the one that matches your interests.

A sampler:

* The Historic Cruise: The ancient civilizations of the Mediterranean, visiting Athens, Egypt, Crete, Rhodes for 13 days aboard the Royal Viking Sea in October.

* The Ritzy Cruise: 60 spacious cabins--and no more--aboard the new Sea Goddess, debuting April 7 for week-long sailing in the Mediterranean and Caribbean. "Intimate," "private" and "exclusive," say the owners, especially at $3,300 a person per week.

* The Long Cruise: 63 days encircling the continent of South America aboard a 100-passenger freighter, sailing regularly from Vancouver on the Delta Line (south through the Panama Canal and north again via the Strait of Magellan). Seventeen ports in 11 countries, with time on shore to explore.

* The Rigorous Cruise: That's how Pearl Cruises describes its 16-day China Explorers Cruise, sailing April, May and June. On shore tours, passengers must walk so much they joke about burning off calories faster than they consume them.

* The Gender Gap Cruise: "Distinguished gentlemen," says the Royal Cruise Line, serve as "dining, dancing and card-playing" hosts and shore escorts for single women passengers, "particularly those over 50." Play no favorites, the hosts are told. Find them aboard the Golden Odyssey and Royal Odyssey sailing the Mediterranean and Scandinavia from spring to fall.

* The Nostalgia Cruise: Steamboating on the Mississippi and Ohio, reliving a lively chapter of the American past on the Mississippi Queen and the Delta Queen. Two to 10-night itineraries year-round to the tune of a Dixie calliope.

* The Fitness Cruise: "Fit With Fun," claims the S.S. Norway, the largest cruise ship in the world. The space is needed for three pools, a racketball court, basketball, volleyball, golf-putting, golf-driving, a 1/6 mile jogging track, shuffleboard, Ping-Pong--along with tournaments, classes and lectures.

* The Casual Cruise: Family-style meals and informal dress aboard the American Canadian Line's new 80-passenger mini-cruise ship, Caribbean Prince. Eight 12-day trips around Jamaica this winter, where, says a spokesman, the scenery eliminates any need "for shuffleboard or Sinatra."

* The Quickie Cruise: A one-night getaway out of Tampa for a Halloween Masquerade Ball (Oct. 28) on the S.S. Veracruz. Only $79 to $89 per person, with prizes for the best costume.

* The Pioneer Cruise: Through the "Northwest Passage," a first for a passenger ship, so far as the crew of the ice-breaking M.S. Lindblad Explorer knows. Thirty-six days west from Greenland across the Arctic Ocean to Alaska, the Bering Sea and Yokohama. Sold out a year in advance for August of 1984, but a 1985 repeat is being planned.

* The Romantic Cruise: Almost any cruise, really, but a week sailing the Hawaiian Islands should really stir the heart. Oahu, Kauai, Maui, Hilo and Kona weekly on the ships of American Hawaii Cruises.