Hogtown Creek at Alfred A. Ring Park in Gainesville is closed for now because of high levels of fecal contamination, officials say.
Fecal bacteria can serve as a pathway for serious diseases, primarily to those with open wounds.
Chris Bird, director of the Alachua County Environmental Protection Department, said the appearance of the water isn't an indicator of any possible danger.
"A lot of people really get deceived because it looks like very clean water," Bird said, "but it's the part you can't see that's really harmful."
Savannah Beck and Gregoire Boucher were looking for shark teeth in the creek Monday when they stumbled upon a laminated paper sign warning of the bacteria.
"I was pretty shocked," Boucher said. "Also, it's the only sign that you see. There is no other sign around the park.
"If it is something very important — and it is — you should put signs everywhere just saying, 'People, don't go in the water now.'"