Getty Images

While you're bobbing in the surf, sickness-causing pathogens may be enjoying the water alongside you. Yes, public health organizations are doing their best to test the safety of your swimming water, but that's no guarantee your beach will be closed the minute bacteria show up to ruin the fun.

"It takes time to test water samples, and we don't test every day," explains Jon Devine, a senior attorney with the Natural Resource Defense Council (NRDC), which keeps an eye on your water if you live on either of the coasts, the Gulf, or one of the Great Lakes. Devine says there are also debates among scientists about what constitutes "safe" levels of bacteria.

Why should you worry about any of this? The (often invisible) gunk floating in your water can cause everything from pink eye and stomach flu to hepatitis and meningitis, Devine says. Not even the sand is safe: A recent study in the American Journal of Epidemiology found beachgoers who dug in the sand were more likely to get sick. The authors say sand absorbs all the same pollutants water does. But unlike water, sand isn't replaced by fresh rain or diluted by streams. (So skip the sandcastles?)

To protect yourself from pollution, Devine recommends visiting the NRDC's site, where you can look up water reports for your favorite beach. "That'll give you a snapshot of what your water quality has looked like in the past," he says. Chances are good if the water is filthy, so is the sand, the study above suggests.

But you don't need chemistry to tell you if hitting the waves is a bad idea. Here are five signs your beach is bad news.

1. It just rained. Storm-water runoff is one of the top sources of water pollution, Devine says. If a big thunderstorm pummels your area, staying out of the water for at least 24 hours is a smart idea, he advises, adding, "Seventy-two hours is even better."

2. You see gray. Take a look around your beach. If you see a lot of parking lots, paved roads, and other concrete structures, that's trouble, Devine explains. Because soil acts as a natural water sponge and filter, it helps keep filthy water from running into your favorite swimming area. Concrete and other man-made structures tend to do just the opposite, Devine says.

3. You can wave to the marina workers. Devine says boats discharge all kinds of gross stuff, from raw sewage to gasoline. Also, marinas tend to be located in calm, protected inlets, where the same water can linger for days, collecting pollutants. Swimming in open waters, which tend to be cooler and choppier, is a better idea, Devine adds.

4. Pipes are present. A lot of cities and towns have water collection systems that discharge everything but sewage directly into the local waters, Devine explains. Just look for the pipes, which typically run right up to (or even onto) the beach before disappearing underground, he says.

5. You're bumping into other swimmers. People are dirty. And the more of them you see around you in the water, the more likely you are to encounter illness-related bacteria as a result of "bather shedding," explains Liz Purchia, an EPA spokesperson.