It was a nasty day for a monster watch on the Chesapeake Bay. Cold and windy, it was beginning to feel a lot like Christmas. Especially on the bow of a 31-foot sailboat, pointed into the wind near the mouth of the Potomac River, where Chessie, the legendary sea serpent of the Chesapeake, had allegedly appeared.

"I didn't get a good enough look to tell you exactly what it was," said the captain of our sailing ship, a seemingly clear-eyed business executive who claims to have spotted the beast in late August, but has not the courage to let me use his name. "I can tell you what it wasn't. That was no fish."

Scotland has its Loch Ness Monster. The Himalayas hide an Abominable Snowman. The Turnpike State boasts a Jersey Devil. Not to be outdone, the Chesapeake Bay, seafood capital of the nation, claims its own marine monster -- 40 feet long, with three humps on its back and a head shaped like a football.

There have been sightings of a Chessie-shaped beast in the Bay for at least half a century. But those reports have increased dramatically since 1980, when a North Beach, Md., woman, Helen Jones, said she encountered Chessie while crabbing.

"This great big thing come up out of the water. I could have reached out and touched it with my hand. It was about as round as a watermelon, more brownish than gray and had white spots on the hump," said Jones.

There have been so many sightings of Chessie since then that the Maryland Department of Natural Resources Police formed a "Chessie Patrol" in 1982. The most recent reported sighting occurred in August, when Harry Lohman of Stevensville, Md., said he saw Chessie twice during a three-day period.

"When we came out of the Wye River, my wife said, 'What's that over there?' I said that's a log. She said, 'If that's a log, it's moving awfully fast.' I don't know what it is. But it's here."

The most celebrated sighting of Chessie occurred two years ago in shallow water off Kent Island, Md. What made this report more intriguing than the others was the three-minute color film that Robert and Karen Frew took of the monster. Scientists at the Smithsonian Institution reported the film appeared to be authentic, but refused to identify the subject.

"We believe there was an animate thing there, but we couldn't identify it," said one researcher.

Scientists are understandably hesitant to risk reputations on the testimony of amateur wildlife spotters or the blurred evidence of unfocused film. That natural skepticism is periodically reinforced by reports of strange beasts that turn out to be thoroughly familiar creatures.

A few years ago in India, fishermen reported the discovery of a five-ton sea creature in the Bay of Bengal that had the ears, eyes and mouth of an elephant and a 27-foot tail. When scientists studied the beast, they determined it actually was a badly decomposed sperm whale.

Still, there is much historical evidence to indicate that there may be many creatures on land and under water that have yet to be seen. Since the beginning of the 19th Century, dozens of species that were either unknown or thought to be extinct, such as the white rhinoceros, the pygmy hippopotamus, the okapi and Komodo dragon have been found.

"Nowadays, to say you believe that in some parts of the world there may be quite large animals unknown to science is tantamount to admitting that you are slightly weak-minded . . . It seems to me a very odd attitude to adopt, when you consider the vast areas of the world that, although they may be adequately mapped, are inhabited by only a scattered handful of people, most of whom have little knowledge of, and little interest in, the local natural history," wrote the noted naturalist Gerald Durell 25 years ago in an introduction to a book titled, "On the Track of Unknown Animals."

The similarity in the descriptions of Chessie during the last four years is intriguing. Almost all observers describe the beast as having the circumference of a telephone pole and a length of more than 30 feet. Because most sightings have occurred during the period from late spring until the end of summer, there is speculation that Chessie follows the bluefish run into the Chesapeake and leaves when they depart.

Harry Lohman thinks Chessie might be a descendant of South American anaconda snakes that were placed in the holds of sailing ships in the 18th and 19th centuries to control rats.

"We have freaks. People who are 8 feet tall and weigh 650 pounds," Lohman said. "I guess the only way we'll ever know for sure is if they catch the thing. In a way, I hope we don't."