Every year, at about this time, an estimated 825,000 small-boats sailors in the United States and Canada lay up their boats for the winter. North of the 38th parallel of latitude, the season is over. Except for the "frosbite" fleet, bosts under 30 feet in length are regretfully interned for the winter on a wooden cradle in a boatyard.

But more and more small-bosts sailors are going south on vacation to find another sailing season. Their own bosts being too small to make the trip, they seek one out when they get there. The result is a demand for rental sailbosts that has to be one of the great growth potentials of the tourism industry.

Curiously, it is almost wholly unexploited. There are plenty of cruising yachts for charter, 35-50 feet in length (and longer), costing around $1,000 a week. But a skipper who wants to daysail, or who is on a modest budget, or whose spouse is not sail-minded, has a problem.

You would think that in the British Virgin Islands, if anywhere, a bost in the 18-25 feet range - something you could handle alone - would be readily available. These sailing waters, east of Puerto Rico and the American Virgins, are some of the best in the world, with good, reliable breezes (at least in winter), sheltered moorings and few seious hazards to navigation.

Indeed, the BVI claims to be the barebost charter capital of the Caribbean. But not in the small-boat field. I have just made a survey of hotels and charter companies here, and hae found only one - the Bitter End Yacht Club - which offers a substantial fleet of professionally-kept small bosts for daysailing.

Virtually every resort hotel in the Caribbean, of course, advertises what it calls "sailing." Invariably a graceful sailing bost is shown under way on the cover of the hotel brochure.But when you get there, the sloop or yawl in the brochure proves to have been just posing for a photographer. What are likely to be available to you are Sunfish, Sailfish or some equivalent - essentially surfboards equipped with a sail. They are sporty, but they aren't what you have in mind. When there actually are one or two conventional small sailbosts, they are likely to be beat up, held together with cotter pins and/or priced at $40-$50 a day.

Only one hotel on the main BVI island of Tortola, so far as I have been able to discover, has small sailboats, other than Sunfish, for rent. It is Prospect Reef, where several between 14 and 19 feet long are offered. But they are not moored at the hotel. You have to take a bus or cab about five miles out of town to get to the small lagoon where they are sailed. They cost $30 a day and up. Biras Creek Hotel, here on Virgin Gorda, offers four sailbosts between 19 and 26 feet in reasonably good condition, costing $25-$48 a day. A 14-feet sailing dinghy is free.