When the Coast Guard blocked the cruise ship Norway from sailing last month, the vacations of close to 2,000 passengers went down the drain.

Norwegian Cruise Line had to cancel a seven-night cruise from Miami when inspectors decided its sprinkler system might not work in a fire. The cancellation, on what should have been the second day of the cruise, left passengers to disembark with their luggage in tow in search of a new vacation.

The incident dramatized the fact that cruises carry a risk that land resorts don't. When ships break down, collide, run aground or are stopped from sailing by regulators, hard-won vacation time goes to waste.

Mechanical problems are just one type of letdown for cruise passengers. In recent years, several lines have gone bankrupt, some as their ships were in mid-cruise. And others have scrubbed prized inaugural voyages because new vessels were delivered late from shipyards.

All of which raises the question of whether passengers can judge how likely a cruise ship or cruise line is to disappoint them. Experts cite some rules of thumb. In general both very old and very new ships tend to have more problems. Cruise lines in financial straits are another risk factor, as are ships that get low scores on sanitary inspections by the U.S. Public Health Service.

In most cases, the best thing passengers can do is book a cruise through a knowledgeable travel agent committed to their welfare, buy travel insurance that covers the price of their ticket and avoid unfamiliar cruise lines.

ROUGH SEAS

Even storms can hit cruises harder than land-based resorts.

Bruce Armitage and Leo Xarras were booked on a 10-night Mediterranean cruise on April 27 but when they arrived in Lisbon, Portugal, they were told the seas were too rough to sail. They decided to take a train to meet the ship at its first port of call. Plans changed several times.

Eventually, the cruise was canceled and the passengers sent to Paris as compensation. But the two New England businessmen said they were misinformed about where the ship would go and missed it at several French ports. Arriving in Paris, they said the cruise line told them they were on their own to get home.

Brad Ball, a spokesman for the Fort Lauderdale-based cruise line, said that when the Armitage-Xarras party reached Paris, they asked for an "exorbitant" refund and were refused.

Ball said it was a rare situation. "This is the first time in three years that a cruise was canceledbecause of weather," he said.

Travel agents say it is very difficult to anticipate such problems. "I really wouldn't know how to tell someone what's not a good cruise line," said Gwen Tannenbaum, a travel agent at Olwell Travel in Fort Lauderdale.

Unlike the airlines, which report to the U.S. Department of Transportation, cruise lines don't file reports that federal agencies compile into easy-to-use statistics on cruise reliability.

Some data are available. The U.S. Coast Guard posts the results of its safety inspections of ships calling on U.S. ports on its Internet site. Although the reports are highly technical, a consumer can see if the Coast Guard has found one or more "deficiencies" in an inspection, which the cruise line is obliged to correct at its earliest opportunity.

SAFETY FIRST

Only three ships in the past eight years, the Queen Elizabeth 2 in 1994, the Oceanic in 1998 and the Norway last month, were found so unsafe that the Coast Guard stopped them from sailing.

In each case, the ship in question was more than 25 years old.

While old age isn't an ironclad reason to avoid a ship, it does make them more prone to maintenance problems. Older ships also sometimes have retrofitted safety features, such as the leaky sprinkler system on the Norway that prevented it from sailing. It was installed on the 40-year-old ship in 1998, as required by tighter international safety standards passed by the International Maritime Organization in 1993.