A federal survey has found that the biggest threats at the beach are the ones you can't see. After collecting samples from almost 3,500 beaches along the Great Lakes and marine coasts, the Natural Resources Defense Council states in its annual beach report that one in 10 American beaches are unsafe for swimmers based on water quality testing.

The areas with the most polluted beaches, which overwhelmingly line Lakes Erie and Ontario, need to shore up treatment of stormwater runoff and sewage overflow, NRDC program director Steve Fleischli told USA Today. If insufficiently disinfected, these improperly funneled waters (with stormwater runoff being the biggest offender) flush unsafe bacteria into swimming holes, plus all the diseases that come with them: stomach flu, pinkeye, and hepatitis, to name a few. There's a chance you might even be able to anticipate your illnesses: The EPA found that 3.5 million beachgoers have strokes of bad luck with raw sewage.

Swimmers and sunbathers should especially look out for the 17 "repeat offenders," most of which are in Ohio. These beaches failed to meet public health benchmarks more than 25 percent of the time between 2009 and 2013.

"The [Great Lakes] tend to have a lot of sources of urban pollution and urban slobber," Fleischli said. "Because lakes are basically a closed system, circulation is not as robust, so water could stagnate in those."

The findings aren't all murky, however: The NRDC picked out 35 "superstar beaches" that met water quality standards 98 percent of the time, with "Delmarva" (Delaware, Maryland, and Virginia) having the best-kept beaches.

Now in its 24th year, the NRDC’s beach report analyzed water samples for the first time based on the new Beach Action Value, the Environmental Protection Agency's rejuvenated water safety standards. The BAV has a stricter minimum for water quality, though it introduces no new means of water health measurement. As usual, the BAV requires testing for high bacteria counts that imply waste.

When the NRDC tested water samples this year in line with the BAV, it also tested them against the old standards, and the differential wasn't palatable.

"If we were to compare to the old defunct standard, it would have been about 7 percent of samples [we'd define as unsafe], which tells us we're stagnating in terms of progress of water protection," said Jon Devine, senior attorney for the NRDC.

Our advice? Review the polluted beach listing (the NRDC has a comprehensive searchable map to help you analyze shorelines near you) and support the Clean Water Protection Rule. Go on—the water might be fine.