Those elusive aquatic monsters are surfacing again - or at least reports of them are.
Something of a three-ring media circus has formed around ''Nessie,'' ''Chessie,'' and ''Champ.'' Reputed sightings of these beasts are capturing headlines again, from the North Hero (Vermont) Islander to the Washington Post. ''Chessie'' has even appeared on videotape, giving new credibility to the legends of the ''monsters'' of Loch Ness, Chesapeake Bay, and Lake Champlain.
The videotape of a slender, 35-foot object snaking through the waters of the Chesapeake Bay in May is viewed excitedly by cryptozoologists as possibly the most convincing documentation so far of unexplained water phenomenon, says Joe Zarzinsky, who heads the Lake Champlain Phenomenon Investigation and is a member of the newly formed International Society of Cryptozoology.
The three-minute videotape, which is to be examined next month by scientists at the Smithsonian Institution, was taken by Bob Frew, a businessman from Kent Island, Md. It shows the dark, serpent-like creature from a distance of 200 yards and includes an audio recording of his children squealing with delight as the creature passes a few feet from swimmers.
Explanations for the identity of ''Nessie,'' ''Chessie,'' and ''Champ'' range from prehistoric creatures and optical illusions to snakes or turtles or inanimate logs. Some scientists suggest that what the Frews and others for centuries have been seeing in the lakes and bays of the world are a surviving population of some prehistoric animal - perhaps unchanged descendants of the zeuglodon, a mammalian ancestor of today's whale which propelled itself by vertically flexing its spine.
Others suggest these animals could be related to the prehistoric plesiosaur - a reptilian relative of the dinosaur.
Asked if what she saw in the Chesapeake Bay might have been something as explainable as a distorted view of a turtle Mrs. Frew adamantly retorts, ''What I saw was no turtle.''
The Frew tape is just one of this summer's sitings of unidentified swimming objects. Sitings seem to rise with the temperature, peaking during the summer months when more people are on the water. The Enigma Society, a Baltimore-based group interested in unexplained phenomena of all sorts, reports several sitings of ''Chessie'' since the May 31 Frew tape was made. Five ''Champ'' sitings have been documented by Mr. Zarzinsky this summer (19 last summer) at Lake Champlain in New York and Vermont. The latest, on July 19, was made by a New York animal warden whose camera failed him at the moment ''Champ'' appeared.
But if there has been any monster lurking about this summer, says Enigma Project spokesman Bob Lazzara, ''it's been the press.'' As publicity trickled out of tiny Kent Island this summer, the Frew's beach-front home was besieged by television crews, newspaper reporters, and sightseers.
Lazzara says he's concerned that legitimate sitings of Chessie, Nessie, or Champ could go unreported if ''closet witnesses'' think they won't be taken seriously or will be hounded by the press.
Mrs. Frew told the Monitor she wouldn't have reported the videotape if she had known what the media response would be.