WHEN SHE slithered into the tub, silky jets of hot green water caressed the tense muscles at the back of her neck. The man next to her leaned back his head, shut his eyes and let the soothing bubbles shimmer down his back and legs. Only naked shoulders could be seen and sighs of "oh it feels so good . . . it's so delightful" were heard.

What sounds like the opening of a pornographic novel is not.

It's the newest gimmick from the laid-back land of California. Hot tubs have arrived and are steaming in Washington.

One of the people to get into the act is Californian Len Saxon who recently opened Hollywood Hot Tubs in Columbia (997-3311). For $2,500 plus a $550 installation fee, Saxon says he will set up a 6-by-4-foot redwood tub complete with benches, pump, water filter, heater and hydro-massage unit in your backyard or basement in five hours.

Saxon's competitor, Hot Tubs International in Falls Church, has a rather steamy glimmick. It has printed up a 63-page, X-rated brochure filed with happy hot tubbers in the buff. However, the owner of Hot Tubs International, Charlie Edwards, is not abushed. He says he only gives the brochure to intimate customers. Besides he thinks hot tubbing is "primarily a nude thing. If people first enter the tub with a bathing suit, it won't last. They will take it offi after a while . . . Nudity is not a big deal. Anyway wearing a bathing suit in a hot tub is like wearing a rain coat in the shower."

Edwards has been open since May and is surprised by the enthusiastic reaction, but would not reveal how many tubs have been sold. Business, he says, has been good and "it didn't take long for Washington to get it into their idea of life."

The redwood tubs looks like wine vats. In fact the original tubs in California were. During the 1950s modern-day hot-tubbing was discovered in West Coast vineyards. After an exhausting day of stepping on grapes in a wine vat the stompers would gather at a distant hot spring. Eventually it was discovered that if enough hot water was added to the wine vat, the stompers and the vat could be cleaned together. "One thing led to another, as it usually does in California, and soon a brand new cult was flourishing," says the Hot Tubs International brochure."

The construction of the tanks has not changed much from its wine vat predecessor. The redwood planks are cut slightly concave and the edges are bevelled so each stave fits snugly with its neighbor. There is no glue or fastening needed other than wire hoops which are placed around the tub.

After the tub is set up, the inside is washed with soda ash so the red dye from the wood doesn't get into your skin. When the tub is first filled with chlorinated water it leaks until the wood swells enough to fill the holes. The water is then heated to 105 degrees, the jets are turned on and you jump in en familla or whatever. Don't be disturbed that you may be sharing stale bath water - the water is coninually filtered, like a swimming pool. Most people change the water every six months or so.

The redwood won't rot or get soggy, or as Saxon explains as he lovingly pats the tub like a used car salesman. "It will last for 50 years. There isn't anything better than redwood." And, he says in a conspiratorial whisper, "Redwood isn't getting any cheaper, either. Pretty soon these tanks are going to be a made out of mahogany. It's good wood but not as good as redwood."

Saxon, who became interested in the tubs while living in California, points out that unlike swimmings pools hot tubs can be taken with you when you move. However, empty the water because a 6-by-4-foot-deep tank weighs 7,000 pounds. Another thing, don't put one on the second floor of your house. They should be installed in a basement or outdoors. For outdoors, you can buy a big plexiglas dome that can be heated in the winter. Which brings us to another problem. Washington doesn't share California's climate so if you don't want to turn your tank into a miniature skating rink it should be emptied in the winter or taken indoors.

Some people believe that soaking in hot water is therapeutic. Jets of whirling water have long been touted to help relax tense aching muscles and aid circulation.

But the tax and health benefits are not the selling points. Sex is. The Hollywood Hot Tubs brochure depicts a lovingly unclad couple gazing into the stars while sipping a bottle of champagne. Even the name of the smaller model, the "cuddle tub," conjures up romantic thoughts.

Most people can't stand the heat for more than 15 minutes. The temperature can be regulated and the jets turned off so you can stay in until your skin shrivels up.

California Cooperage hot tubs and accessories are available by writing for the brochure (California Cooperage, P.O. Box E, San Luis Obispo, Calif. 93406). The prices run from $1,350 for the snuggle tub to $2,370 for the 8-footer. However, these are the prices for California residents and prices will be higher in Washington. Edward most inexpensive model is a do-it-yourself kit that sells for $1,600. The kit sells for $100 less in California because the redwood comes from California.

Quite a few places in the Washington area sell hot tubs for California companies, including Stevens, Herbert & William in Suitland and the Columbia Garden Center. Stevens, Herbert & William are agents for American Tank's Mill.

Greg Williams of Chesapeake Hot Tubs to Timonium, Md., admires Cooperage's rather flamboyant selling techniques. At a hot-tub convention last year, he said, California Cooperage displayed their tank surrounded by four Hollywood starlets. When the crowd was sufficient the nubile beauties apparently tore off their clothes and jumped into the tub, creating quite a stir. One elderly woman was rather shocked so she threw their clothes in after them. The starlets put their wet clothes on and got out of the tub, but the clingy sheer covering only made them more attractive. Willis said it was tacky but it worked.

A number of hot-tub dealers have surfaced and then quickly sunk in the Washington area, so carefully check the reputation of the company you're buying from. Willis also warns that there are "quite a few guys in Washington who want to make a quick buck selling hot tubs, so watch out." It is a good idea to check with the Better Business Bureau. Tubs can crack and leak if not properly made and protected in the winter. And making hot tubs is a bit tricky. A stave that is not cut on the vertical grain of the wood will swell the wrong way, causing cracks and splinters, nasty on naked skin. It is important that the staves warp into each joint for a solid tub. Also check guarantees on the wood.

According to Willis it is difficult to get a strong long-time guarantee on tubs from California because the manufacturers feel that once the tubs are out of their territory they can't control what happens to the wood. However, most of the workmanship and accessories (pump, heater, filter, etc.) are guaranteed. Also be sure to get information on how to keep your tub in the winter and while on long vacations. It is advisable not to keep the tank empty for more than a few days.