It appears that when you go to the beach, the sand is more dangerous than the water:

bacteria in water "die, disperse, dilute," says biologist Richard Whitman, station chief at the Lake Michigan Ecological Research Station of the U.S. Geological Survey. But E. coli found in bird droppings and human waste can attach and flourish in sand, he says. "This stuff can last for weeks and months."

Whitman has done studies, including one in 2003 that found bacteria levels in sand at a freshwater beach in Chicago averaged up to 10 times that of the swimming water. The city replaced the contaminated sand --- but within two weeks, the bacteria level was similar to that before the sand was changed.

(Yancey, "Beach sand can be a petri dish of bacteria", USA Today, Jul.21)