Bright days of promise are with us often now, as stark winter gives way to spring. Even the air seems swollen with hope.

It's tempting, with the sun hanging in a blue sky and soft breezes on our cheeks, to believe that the world has turned warm forever and to put away the cautions of winter.

For those of us who venture onto water in small boats -- jon boats, sailboats, canoes and kayaks -- it's a demonic temptation: The water is icy still, and will stay that way for weeks, perhaps into May.

If you tumble into the cold water without adequate protection, it will sap your body heat quickly -- 32 times more quickly than cold air -- and leave you unable to help yourself or to respond to help from others.

The American Canoe Association reports that half of all canoeing deaths occur in this early part of the season, when the water is still icy. Whenever the combined air and water temperature is less than 100o F., the ACA says, "consider the conditions severe" and take all winter precautions including the use of wet suits similar to those divers wear.

In hypothermia, the technical name for the sequence that occurs as the body's internal temperature falls, muscle coordination vanishes; the mind numbs and speech becomes incoherent; there is uncontrolled shivering; and then comes unconsciousness -- and, without proper treatment, death.

Exertion speeds heat loss, and wearing a life jacket or "float coat" reduces your need to struggle or work to stay afloat. In a short time, the cold water would destroy your ability to tread water or swim anyway. A vest-type life jacket also doubles as an insulator.

My first brush with hypothermia was in the rescue of two young men who had swamped their canoe on the Cacapon River one bitter March day. They had been in the water only minutes but were too weak to walk 10 feet to shore without help.

The second encounter was more personal: It was on a bright, hot day in July, but the river water came from the bottom of a deep mountain lake at about 46o F. My husband and I wiped out in our canoe at the top of a long rapid. Though he got to shore immediately, I was in the water several minutes before friends could pull me out.

Insisting that I was okay, I got back in the boat and headed into the next rapid, but nothing worked. I had no strength and little coordination and I began to tremble. Wiser friends, recognizing the symptoms, forced me ashore and into a regime of warm fluids, rest and warming in the hot sun.

The only thing that will prevent the loss of body heat and keep you safe for a long time in the water is the diver type of neoprene rubber wet suit. The closed-cell neoprene keeps you so warm the water feels good.

Paddlers began adopting wet suits years ago. Surfboarders and, lately, windsurfers also use special versions of wet suits for protection. Hunters and fisherman who use small boats and cold-water sailors have equal reason to explore the use of wet suits, especially some of the new, ultrathin styles.

A traditional diver's suit has several drawbacks for non-divers: Its quarter-inch thickness is uncomfortable for long cramped sitting and the tight fit is too restrictive in the shoulders, arms and knees. Several manufacturers now advertise a "canoe cut" and one, Sea Suits of California, makes an entire line of suits, called Wilderness, designed specifically for paddlers.

Sea Suits and several others also offer suits designed especially for surboarders and windsurfers. Hunters, fishermen and sailors may find these specialty lines useful "insurance."

For paddling, I go for the long, sleeveless farmer john which can be worn with a wet-suit jacket on very cold days or with a wool sweater and waterproof paddle jacket in milder conditions. They present one obstacle: You have limit your fluid intake or develope a large carrying capacity.

The cost of wet suits runs from about $50 for a shortie model to as much as $300 for a custom-fitted one with lots of extras. A farmer john is $70 to $80. With reasonable care, it'll last five to 10 years, which makes pretty cheap life insurance.

If you need one only once or twice a year, for an annual duck hunt or a Boy Scout canoe trip, you can rent a diver's suit at several local dive shops for between $10 and $14 a day.

If you buy a wet suit, there are three things to consider: fit, construction and rubber quality.

* The suit must fit snugly around the trunk to do its job.

* Seams should be sewn on at least one side, preferably with double, interlocking stitching. Avoid seams that are glued only, even if they have seam tape over them.

* Unless you plan to swim the Cheat River Canyon, 1/8i or 3/16i thick rubber is fine. You can get nylon linings on one or both sides. Plain neoprene is warmest, but harder to put on. Nylon inside makes getting in and out easier; nylon outside protects the rubber from snags and gouges. Nylon on both sides reduces warmth for the same thickness of rubber.

Two types of rubber are used -- "gas blown" or "chemically blown," which refers to how the gas-filled, closed-cell structure is formed. There is overwhelming agreement among retailers that the gas blown lasts longer.

Sea Suits' Wilderness line uses gas-blown material and is available at Hudson Bay Outfitters, Appalachian Outfitters and Springriver Corporation and through a few dive shops that serve paddlers and surfers, including American Watesports and the National Diving Center. For a slight fee you can order a custom-fitted suit from any of them.

They all offer, also, neoprene rubber socks, gloves and boots, which range from $10 to about $40 for the thick boot with a grip sole.

Dive shops also carry a full line of maintenance items, including special shampoos and lubricants for the zippers that will not damage the rubber, neoprene cement and scraps of rubbers for repairs. They can also offer invaluable advice on making repairs and maintaining the suit. GOING WITHOUT

If you insist on venturing out on cold water without a wet suit, wear wool and carry extra clothing, fire starters and warm fluid in a waterproof bag -- not a plastic garbage bag.

Wool is the only clothing material that retains some insulating value when wet -- 40 to 60 percent -- and if you are in the water long, it will have no heat-holding value, so you must get out of the cold water quickly.

Those who attempt cold whitewater should understand that the fast currents and turbulence in rapids make it likely that they'll be in the water longer, so they're at the greatest risk without a wet suit.

The American Canoe Association has tips for small-boaters and a safety packet that includes a pamphlet on "Survival in Cold Water," which was prepared in cooperation with the Coast Guard and the Pennsylvania Fish Commission. You can get them free by writing ACA, P.O. Box 248, Lorton, Virginia, 22079.