Just when you thought it was safe to go back in the water . . .

It is, "Jaws" notwithstanding.

"The reality," says biologist Charles Kingsley Levy, "is that more people die every year eating clams than die from shark attacks."

At the risk of making vacationers paranoid -- which is not his intent -- Levy does has a list of "dangerous" insects, spiders, fish, snakes, and wild animals that could be lurking in the mountains, forests, lakes and beaches.

The most dangerous creatures of summer, says Levy, are the Hymenoptera, the insect order including wasps, hornets, bees and ants, species that can inflict painful -- and occasionally fatal -- stings.

"The animals we fear most," Levy soothes, "usually do the least harm. The things that really hunt out humans are more annoying than deadly, but they're the things that cause us the greatest annoyance." And those annoyances -- black flies, biting midges, sand flies, mosquitoes, chiggers, itch mites, fleas -- are in the greatest number.

Levy confirms that the itch from these bites is "infinitely worse" at night. As he explains the body hormone cortisone (from the adrenal cortex) that controls allergy inflammation follows a 24-hour cycle: "It waxes and wanes." In the early part of the evening, your anti-inflammatory substances are at an ebb ("this is when the itch is maddening"). By early morning, "even before you get up," your cortisone level starts coming back and the itches aren't so bad.

Also in Levy's A Field Guide to Dangerous Animals of North America (1983, Stephen Greene Press, 164 pp., $9.95) are these indigenous creates and Levy's recommendations for prevention and treatment of their bites and stings:

* Chiggers, ticks, mosquitoes and biting/sucking flies. Effectively controlled by repellents containing "deet." Muskol (95 percent "deet"), Repel (52 percent), (Cutters (31 percent) and Deepwoods Off (29 percent) are among the more effective brands. Some plastic or synthetic fabrics can be damaged by "deet."

When going into the field, wear long, loose pants and sturdy footwear. Taping the pants ends over socks and spraying with repellent is recommended. After returning from the field, shake off clothes and/or wash them in hot soapy water. Use nail-polish remover or kerosene to remove imbedded ticks. Steroid cream and local anesthetic creams should relieve painful irritated insect bites.

* Sea nettles. Never fatal. Mildly painful stinging sensation and red rashlike appearance. More annoying than dangerous. Scrape off stinging cells ("teeny little balls with hairlike hypodermic needles") with a dull blade. Ammonia and meat tenderizer will soothe the affected area. Rubbing sand on the sting is "probably the worst thing you can do."

* Wasps, hornets and bees. Sometimes fatal. Wear long-sleeved shirts and pants; avoid bright floral prints; don't use sweet-smelling cosmetics or after-shave during outdoor activities.

Reactions: instant and continuing pain, and a small red swelling, usually last a few hours. "If the swelling starts to increase between the sting and the heart, that's an indication you have a sensitivity. If your eyes start to water and your nose runs and you break out in hives and the swelling continues, or if you start to wheeze and have respiratory problems, that should be treated as a medical emergency and you immediately should get yourself to an emergency room."

* Copperhead snakes. Seldom fatal.Usually painful, with considerable swelling. Herpetologist Dr. Findlay Russell recommends "a conservative approach to treatment" and says physicians should "withhold antivenin unless symptoms indicate severe envenomation or if the victim is either very old or very young."

Copperheads, says Levy, are "small, attractive and abundant, but they're relatively shy. If people follow the rules in the book -- don't step over logs or rocks; step on them and then over; stay alert -- nobody should get bitten.

It is human beings, however, who top the list in frequency of attacks. And human bites are the third most common type of bite wound, after dogs and cats.

Asserts Levy: "You're a lot safer in a national park than you are in Central Park at night . . . The flora of the human mouth is diverse and a neglected bite wound can have serious complications, including amputation.

"In parts of South America, it isn't the poisonous snakes or frogs that you worry about. It's the guerrillas and the government troops. I don't know which are worse."