Sailing time is always exciting. You've already checked out your cabin, made a pass-through of the ship and gotten an idea where everything is located, and now you're on deck waiting for that magic moment when your cruise will begin.

Usually there's a band playing. You probably have a cool drink in hand and you've already started to make friends with fellow passengers standing next to you at the rail. Some sun worshipers already have stretched out on the deck chairs by the pool.

Then comes the mighty blast of the ship's horn and slowly the cruise liner edges away from the dock. You and your companions wave frantically to friends and relatives on the dock who have come to see you off. Soon, as the figures grow smaller, the ship starts to slice with increasing speed through the harbor channel. Ahead lies that neat seam between sky and water called the horizon and, beyond it, the promise of days and nights of fun at sea and in exotic ports of call.

They call this special kind of vacation experience a cruise, and it comes in many forms. Some cruises are quick round trips to nearby islands, others are weeks-long voyages to far-off destinations. Some are luxurious, others less so. Some make only a few port calls, others make as many as they can within their allotted time span.

This year, your choice of cruises promises to be more varied than ever as the industry continues to expand. You'll also find it easier to book the cruise of your choice, for two reasons: there are more ships and the recession is still being felt in the cruise industry. That means more berths are empty these days.

It also means that you, the passenger, are in the driver's seat now. A couple of years ago, you had to get on a months-long waiting list to book the cruise of your choice, and a lot of cruise lines were simply order-takers. The tables are turned this year, however. The recession put a crimp in the cruise business just as new and expanded ships came on the market, increasing the total cruise capacity at a time when demand was slackening.

The result? It's a buyer's market. You can pick and choose among cruises, and waiting lists are relatively short, though for more desirable cabins and-or price categories you may still have to book well in advance.

On top of that, new ships will be sailing into port with a cargo of different destinations and experiences. More new liners are being built or converted than ever before and many of them will begin boarding passengers within the next year or so.

In Miami, Carnival Cruise Lines is bringing in its new 30,000-ton Tropicale this fall and will operate it out of Miami next winter before shifting it to the California-Mexico arena. It is also about to sign a contract to build still another new vessel.

Scandinavian World Cruises will inaugurate its daily Miami-Freeport service in December after it refurbishes the former Caribe this fall and renames it the Scandinavian Sun. Its second ship, the Scandinavia, is currently being constructed in France and is to begin cruises between New York and Freeport next year. Meanwhile, Commodore Cruise Line, which had sold its Caribe to Scandinavian, hopes to have a larger replacement ship operating out of Miami by December.

Possibly the most significant trend in cruising this year is toward shorter cruises. The coming return of the one-day round-trip to Freeport from Miami is one example. Another is Royal Caribbean Cruise Lines' decision to shift the Sun Viking's 14-day schedult to seven-day cruises as of April 5. The Sun Viking, which had made 14-day cruises the entire eight years of its existence, will have two distinct one-week itineraries (Port-au-Prince, Port Antonio, Grand Cayman and Cozumel one week, and Nassau, San Juan and St. Thomas the alternate week).

The three- and four-day market also is being augmented this year in South Florida. Currently, four vessels make the twice-weekly cruises to the nearby Bahamas. But within 30 days, announcement is expected of another two ships that will enter the Monday-Friday market out of Miami. And starting April 3, Costa Lines' Amerikanis will begin year-round, three- and four-day sailings out of nearby Port Everglades.

Port Everglades, north of Miami, also has landed two other major cruise liners for winter sailings. Home Lines is bringing its 40,000-ton Oceanic to the Fort Lauderdale port this December to make seven and 14-day winter cruises out of Florida for the first time. And in the following year, the line will base its brand new 30,000-ton Atlantic at Port Everglades for winter cruises.

Royal Caribbean also has a new ship in the making -- known now only as No. 431 -- which it expects to put into operation in Miami in October 1982. And both Sitmar and Holland America have announced plans to build new ships for delivery in 1983.

While the Caribbean is still the favored destination for summer cruisers, that season is also prime for destinations you can reach only in warm weather.

Summertime Inside Passage cruises to Alaska, for instance, are becoming so popular that the National Park Service has limited the number of cruise ships that can visit spectacular Glacier Bay. Some 14 vessels, plus Alaskan and Canadian ferries, will be cruising in the Inside Passage this summer.

Also becoming more popular are summer cruises on the East Coast. Three small liners -- American Cruise Lines' Independence and Eagle and American Canadian Lines' New Shoreham II -- run cruises from Chesapeake Bay to New England, and some sail up the Hudson River and into the St. Lawrence.

Royal Viking Line will send its Royal Viking Sky on summer cruises from New York to New England and up the St. Lawrence River to Montreal and Quebec and into the steepwalled Saguenay River fjord. Norwegian America's Sagafjord also will sail that way, dropping in at such ports as Halifax and Gaspe. Small cruisers ply the St. Lawrence out of Montreal as well.

River and lake cruises also reach a crescendo in summer. Two well-known ships, for instance, cruise on the Mississippi -- Delta Steamship's Delta Queen and Mississippi Queen.

In Europe, more than a dozen liners cruise through the fabled Greek islands. Some large liners circle the Mediterranean, while others venture north the Baltic and even the North Cape, the top of Norway.

What will it cost you to cruise the year? The days are long gone when you could take a three-day cruise from Miami to Nassau for $59. Today, lowest fares on the three-day-jaunts run about $70 a day, and those cabins are limited. Seven-day cruises run from around $125 to $150 a day per person, double occupancy, for standard stateroom accommodations. The most luxurious lines average around $200 a day.

Of course, the better your accommodations, the steeper the fare. Suites can run into hundreds of dollars a day. Some new cruise ventures are based solely on this kind of luxury travel. Helge Norstaad, former president of Norwegian Caribbean Lines, is organizing a new company that will build four new small cruise liners carrying perhaps 150 passengers. These will carry well-heeled passengers on luxury "yacht-type" cruises.

But even Norstaad's luxury cruises would be eclipsed by a recently announced plan from a company called Transit Risk Corp. Its board chairman, James M. Beasley, claims the firm will build a new 75,000-ton, $495-million vessel, the Titanti II, that will carry only 600 passengers -- but at $1,000 a day.