`Floating hotels' fit for the times. With more people expected to board each year, cruise lines are expanding fleets and specializing amenities to suit the increasing number of singles, families, and professional couples.

SOARING five stories in a glass elevator isn't much in this day of huge hotel atriums. But when it's on a cruise ship, that's news. That and more is in store for passengers on a new generation of ships that promise a world of glamour and amenities undreamed of even during the golden age of transatlantic liners.

Royal Caribbean Cruise Line's (RCCL) new ship, Sovereign of the Seas, will have not only a five-story-high atrium lobby with two glass-enclosed elevators, but also a ship's lounge extending two-and-a-half decks high and seating over 1,000 people. When it comes into service in January, the 1,138-cabin vessel will be the largest cruise ship afloat.

RCCL is not alone in building glamorous mega-ships. More than 9,000 staterooms will come on line in the next three years, due to bigger fleets by Sitmar Cruises, Royal Viking Line, Royal Cruise Line, Holland-America Line, and Carnival Cruise Line.

According to industry sources, the reason for the boom in shipbuilding is simple. ``Only 3.6 percent of the population has ever cruised,'' says Kirk Lanterman, chairman of Cruise Lines International Association. ``That means there's a vast market out there that hasn't been tapped. We expect the number of cruise passengers to increase by more than 15 percent per year in the next four to five years,'' he adds.

Not only are people cruising in greater numbers, but new and different groups of people are cruising. Once thought to be mainly the purview of retired people with lots of time and money to spend, cruise ships today are afloat with young married couples, families, business executives, and singles.

``Approximately 50 percent of our first-time passengers to the Caribbean are under 35,'' says Mr. Lanterman, who is also president of Seattle-based Holland-America Line.

Joe and Rosene Pirello, both in their 30s, are typical of the new cruise passenger. ``We wanted to do something special to celebrate our 10th wedding anniversary,'' says Mr. Pirello, who works for Northwestern Mutual Life in San Diego. The Pirellos chose an 11-day cruise along the Mexican Riviera on Los Angeles-based Sitmar Cruises' 1,200-passenger Fairsky.

``I started relaxing as soon as the ship left the dock,'' remembers Joe. ``I liked not having to worry about anything,'' adds Rosene, a pharmacist.

The phenomenon of the working couple has caused lines to offer more one-week and 10- or 11-day itineraries. ``Working people usually can't take off more than two weeks at a time,'' says Mike Hannan, senior vice-president of marketing services for Princess Cruises in Los Angeles.

Business executives are also attracted to the relaxed atmosphere of a cruise. ``In my work I'm constantly having to make decisions,'' says Don Krieger, partner in Terranomics, a retail brokerage company in San Francisco. ``But on that RCCL cruise, the biggest decision I had to make was whether to have guava juice or apricot nectar at breakfast.''

Cruising is also gaining in popularity among middle income groups. Although some deluxe cruises can cost as much as $520 per person per day, such lines as Premier Cruise Lines and Chandris Fantasy Cruises offer voyages that average less than $150 per person per day. Fares on Admiral Cruise Line, Carnival Cruise Lines, Cunard Line, Home Lines, Norwegian Caribbean Lines, and Royal Caribbean Cruise Line average less than $200 per person per day, and that price includes all meals, entertainment, and activities. Most lines offer considerable discounts for early booking, too.

Single people are attracted to the cruising market because of the camaraderie aboard most ships. At dinner, they are usually seated at large tables, where they can meet a variety of people, and most lines have special parties for single passengers. ``We do everything possible to make singles feel comfortable on our ship,'' says Fernando de Oliveira, cruise director on the Royal Viking Sea. ``We have many group activities people can participate in.''

More families are cruising, too. Reasonable rates are available for a third or fourth person in a cabin, and many lines have special family programs. Sitmar, for instance, provides a trained youth staff and a Youth and Teen Center on each ship. ``Activities are going on all day long - arts and crafts, Italian lessons, and talent shows,'' says Maureen Ricker, youth activities coordinator on Sitmar's Fairsky.

Along with the trend toward younger passengers has come a greater awareness of fitness. Cruise lines still offer a range of practically non-stop dining from early morning breakfast to an elaborate midnight buffet, but most lines now offer low-calorie alternatives to the normally rich cuisine. Most of today's ships have full-service gyms and regular fitness programs. Sitmar's Fairsky, Admiral Cruise Line's Stardancer, and Holland-America's Noordam and Nieuw Amsterdam all have fully equipped weight-training rooms.Norwegian Caribbean Lines has a complete fitness program that includes basketball, volleyball, and afternoons of pool-side games.

Although the major cruise lines are building ever-larger ships to accommodate these active passengers, there is also a concurrent trend to small ships that offer a more intimate cruising experience. Cunard's 100-passenger Sea Goddess I and Sea Goddess II offer elegant cabins, gourmet meals, and exotic itineraries. Windstar Sail Cruises' Wind Star and Wind Song, luxury sailing yachts with teak interiors and computerized sails, cruise to little-known ports in the Caribbean and the islands of Tahiti.

American Cruise Line and Clipper Cruise Line offer historical cruises along the eastern seaboard, and for a taste of Americana, there's the Delta Queen and the Mississippi Queen, paddle-wheelers that ply the waters of the Mississippi and Ohio Rivers.

Whether observing a black bear from the deck of a tiny explorer ship or dancing in a high-tech disco on a new super-liner, today's passenger is finding a richer cruise experience. Many would agree with P.S. Panagopoulos, founder and chairman of Royal Cruise Line, who said, ``Cruising is the best vacation ever designed by man.''