It was a good crossing. The sea calm, and the big ship moved through the night on a beeline from Portland, Maine, toward this quaint town on the western tip of Nova Scotia.

The ship was the M.S. Caribe, and it was demonstrating that a straight line was indeed the shortest distance between two points.

Consider: Portland and Yarmouth are separated by 858 miles of highway. They also are separated by only 200 miles of water. If one drives those 858 miles on the primarily two-lane roads of Maine, New Brunswick and Nova Scotia, one puts in about 20 hours on the road, expends considerable effort and burns up a lot of fuel.

But if a traveler drives his car aboard the Caribe, he will be transported those 200 miles in 10 hours across a stretch of the Bay of Fundy known as the Gulf of Maine. He will have to expend virtually no energy of his own -- and none in his car. During the crossing he will have a wide variety of things to do, from trying his luck in the ship's casino to shopping in a duty-free gift shop.

In the saving of gasoline alone, the Caribe way to Nova Scotia is a travel bargain. During the peak summer season, a family of four with a passenger car can make the crossing for a base fare of about $155.

Cabins, practically a necessity on the night crossing from Portland to Yarmouth, range from $29 for a two-berth cabin on a lower deck to $75 for deluxe double on the restaurant deck. On day crossings, cabins are available for $18 ($50 deluxe). All cabins have full private conveniences.

The Caribe, a sleek, 11,000-ton vessel that can carry about 1,000 passengers and 200 motor vehicles, from cars to buses and tractor-trailer rigs, often is called "the ferry." It is much more. It is an authentic cruise ship, with car decks deep in its innards. During the winters it meanders in the Caribbean.

"What we aim to do," said Henk A. Pols, general manager of Prince of Fundy Cruises, which operates the Caribe under a charter, "is make a crossing a cruise-type experience for our passengers."

The Caribe makes a 24-hour round trip day in and day out, which means that all personnel have to operate constantly at peak efficiency. The ship departs from Portland every day about 9 p.m. and arrives in Yarmouth the next morning about 8 o'clock Atlantic Daylight Time (an hour later than Eastern Daylight Time). Two hours later, it heads back toward Portland, reaching there about 7 p.m. EDT. (The crossing to Yarmouth end in mid-October, when the Caribe will move to Alexandria, Va., for a series of five cruises.)

On day crossing, when the weather is right, a swimming pool is available without charge. Or you can stretch out in lounge chairs and soak up the sun and salt air. There are deck shuffleboard, a feature-length movie and a bingo game during daytime crossings.

Open during both day and night crossings is the casino, with slot machines aplenty, plus an array of gaming tables. Nearby are a piano bar and a lounge in which there is continuous entertainment by two musical combos. There is the Columbus Room for those who like gourmet dining, with prices that in these inflationary times are eye-openers -- filet mignon for $7.95, for instance.

There is a larger restaurant, the Bahamas Room, featuring what is known as "The Bountiful Buffet." The food is beautifully prepared and the selection is extensive, and return trips to the buffet table not only are permitted but are urged. Elsewhere on the ship is a snack bar.

The ship's company of 180 -- a mini-U.N. group, with Germans, Americans, Italians, Canadians and Koreans -- is efficient, cooperative and amiable. The man charged with shaping up those members of the outfit who come in direct contact with the passengers is Bruno Rossi, 31, chief purser, and he does a crackerjack job.

"It's simply a matter of organization," Rossi said. "Everyone not only has specific basic duties but also has to be able to take over somebody's else's duties, should the occasion arise. Also, we have to move fast during the two hours we're in port in either Yarmouth or Portland. Emptying a ship of this size and then making it apple-pie neat for the next sailing within two hours is not easy -- but we do it."