THE CRUISE BUSINESS is floating at high tide this year as "theme" voyage continue to whet travelers' appetites for a vacation at sea. Whether you choose a week's cruise with jazz greats, learn gastronomic secrets or how to play backgammon from experts, up anchor with a symphony orchestra, or perhaps follow an eclipse (typical theme trips); take a longer and more expensive round-the-world adventure or wet your feet with a three-day mini-sail, you'll be in good company.

This Special section, with a boatload of information about embarkations, finds various authors agreeing that nothing could be finer than the life aboard a liner. So if you're a landlubber, be forewarned that their stories may endanger your budget.

If recent news reports about the Panama Canal treaty and the concomitant controversy have sparked your interest in the big ditch, the best way to take a first-hand look is from the deck of a cruise liner.

A daylight transit of the Canal, watching the locks open and close as the "mules" (engines on tracks) pull the vessel through - sometimes with barely enough room to spare - can be unique travel experience.And when combined with a brief shore excursion in Panama and perhaps a tropical deluge, common in the Canal Zone, it makes for an unforgettable day.

But don't expect to find more than half a dozen cruise liners still flying the U.S. flag - and that number may soon drop to only four vessels.

These S.S. Monterey and the S.S. Mariposa, which in fiscal 1977 are being operated by Pacific Far East Lines with a $17.5-million subsidy from the U.S. government, face the loss of those federal funds beginning in December.

Under, law, the statutory economic life of a U.S. merchant vessel is 25 years, and all subsidies must end at that time. Both ships are completing a quarter century of service. Unless Congress extends the payments, the PFEL says the Monterey's last voyage will be a South Pacific cruise in December and the Mariposa's swan song will be a Hawaii cruise in March. The vessels would be sold, probably to a foreign buyer if one can be found.

"Without the subsidy, U.S. cruise ships cannot compete with foreign vessels which . . . also receive government subsidies or other generous forms of tax relief," a spokesman for the line said.

Both vessels were acquired by PFEL in 1971 from the Matson Line, and sail regularly from California.

Four passenger-cargo liners also still fly the American flag. They are the Santa Maria, Santa Mariana, Santa Mercedes and Santa Magdalena, which the Prudential Lines has operated around South America from the West Coast, with boarding also permitted in Curacao and other ports. The vessels recently were sold to Delta Steamship LInes of New Orleans, a subsidiary of Holiday Inns. The sale "is contigent upon approval by various U.S. regulatory bodies," according to Delta, and Prudential still operates the ships. All other cruise ships are registered in foreign countries (see Registry of Ships on Page H6.

Washington travel agent Andy Spielman, vice president of Water Travel and a director of the American Society of Travel Agents, points out an interesting cruise fact that might keep your bank balance from sinking if you and your agent keep it in mind.

Basically, unless a passenger begins the cruise from a U.S. port, explains Spielman, the sailing is not bonded under U.S. law. There are a number of cruises under foreign flags, offering departures from foreign ports in conjunction with fly/cruise programs, which are not required by U.S. law to provide a performance bond. Though cruise lines are generally reliable. Spielman said, there is no guarantee in those cases that any payment you make to the cruise line will be covered by insurance (and refundable to you) in the event the line goes bankrupt.

Spielman, chairman of ASTA's National Maritime Committee, acknowledge that two lines have collapsed in the past few years. In those two cases, the lines were covered by insurance and passengers received full refunds, according to a Maritime Commission spokesman.

Under U.S. law, the only evidence of financial responsibility for performance required from a carrier is intended to cover those passengers who embark at U.S. ports. Thus, it would make no difference if the vessel itself began a cruise from a U.S. port if the passenger boarded at a foreign port for a segment of the cruise.

However, in the case of a subsidized U.S. flag carrier, such as Prudential, knowledgeable sources agree that there is sufficient evidence of financial responsibility to cover any refunds due all passengers in case a voyage is not performed, including those passengers who might have boarded the ship in a foreign port.

Vessels with fewer than 50 passengers, Spielman wrote in a recent ASTA newsletter, "are not covered by the performance bond regardless of port of embarkation."

Citing examples, Spielman explained that passengers' funds are not covered if they embark at Vancouver, Canada, on an Alaska cruise, but they are covered if they fly to Alaska and embark at Skagway, an American port. "Eastbound transaltantic sailing are covered; westbound transaltantic sailings are not . . ."

ASTA is seeking revision of the law and "full protection on the performance bond to American passengers regardless of the port of embarkation," Spielman added, nothing that problems of enforcement outside U.S. jurisdiction make amendment difficult.