1.Sand FliesUnlike mosquitoes, which prick your skin, sand flies (no-see-ums and biting midges, for example) gnaw into you with tiny teeth before sucking up your blood. They don't carry disease but may leave you covered in itchy bumps that last about a week.
Banish bites: Scratching through your skin can lead to infection, so prevent bites by applying a deet-based repellent before you hit the dunes and every time you dry off after a swim.
2.Food PoisoningEvery year, one in six people contract a foodborne illness, according to the Centers for Disease Control and Prevention. And since bacteria breed faster in a warm environment, cases tend to spike during the summer, says Men's Health advisor Ted Epperly, M.D. Symptoms include diarrhea, abdominal cramps, vomiting, and fever.
Pack smart: Stash spoilage-prone foods, like meat and mayo, deepest in your cooler. Eat them within a half hour of taking them out.
3.JellyfishOf 2,000 known jellyfish species, only 70 cause serious harm to humans. If you're stung, expect a painful, burning reddish or purplish rash.
Burn it out: Apply heat, which changes the toxin's protein structure to neutralize pain, says Paul Atkinson, M.B., B.Ch., B.A.O., of Dalhousie University. So scrape the stingers out with a credit card and give the area a 20-minute soak in water just hotter than a hot tub (109°–113°F). And no one has to pee on you.
4.Septic SandE. coli from animals (or rather, their poop) and runoff from sewers and septic tanks can live on warm sand. It's one of the Environmental Protection Agency's (EPA) biggest concerns for beachgoers, says spokeswoman Enesta Jones. If you're infected, expect to have diarrhea within a day.
Cut the crap: Check the EPA's database for beach advisories or closures, and use hand sanitizer before digging into your cooler, says Jones.
5.Rip CurrentsA shark isn't your top underwater enemy. Rip currents are behind more than 80 percent of lifeguard rescues and 100-plus drownings every year, says B. Chris Brewster of the U.S. Lifesaving Association.
Beware the calm: "Rough water is often safer than the calm spots," says Brewster. Caught? If you can't escape by swimming parallel to the shore, go with the current. Once it dissipates, try again.