Greater Manchester Police urged people not to be tempted to cool off in open water as high temperatures continue across the North West this week.
The force said reservoirs were “extremely dangerous and are not safe places to swim”.
“Reservoirs are clearly signposted and swimming is prohibited, yet incidents continue to occur,” it said.
“Even on the hottest days, the water temperature can remain much colder than expected, often only reaching around 12°C to 16°C.
“Cold water shock can happen quickly, affecting your breathing, movement and ability to stay afloat.
“As temperatures rise this week, please stay safe, choose designated swimming locations, and stay out of reservoirs.”
Black added: “Because they are not for public swimming, once you are in a reservoir you’ll find that there is no escape out of it.”
Cheshire Police also warned of the dangers.
“While it might be tempting to cool off in open water and the surface can look safe, it can hide strong undercurrents, shockingly cold temperatures, debris, and disease just a few feet below the surface,” the force said.
“Cold water shock can affect even the strongest swimmers.”