Rotherham Council warned people not to risk their safety by taking a dip this summer.
It tweeted: “However tempting it may be in the hot weather, we’re reminding you not to go swimming in open water even if the water looks calm.
“Strong hidden currents and debris under the water line as well as very cold temperatures can hinder your swimming abilities.”
Yorkshire Water last year revealed how it recorded nearly 8,000 incidents that summer of people swimming or intending to swim at just 18 of its reservoirs, including those at Agden, Broomhead, Dale Dike, Damflask, Redmires, and Rivelin.
Many of those cases involved unaccompanied children.
It said the risks, which include cold water shock, underwater machinery and unseen currents, were often underestimated.