Officers from the Newport Safer Neighbourhood Team made their appeal after being called out to reports of children swimming in a field irrigation reservoir near Edgmond on Tuesday afternoon.
PC Lee Thomas, from the team, said: "We aren't trying to be killjoys, and we certainly understand everyone's desire to find a way to keep cool at the moment, after all we are out there all dressed in black and having to wear stab vests and other equipment as well. But quarries and reservoirs are not safe places for people to swim, young or old.
"Although on the surface, jumping in it may seem an appealing option, you just don't know what lies beneath, how deep or how cold the water actually is; it may be submerged weeds that you run the risk of becoming entangled in, or even old abandoned machinery or items that have been dumped, leading to the risk of sharp, exposed metal that cause cause serious injury or worse.
"In addition, you don't know what contaminants or toxic materials might have been released into the water from items that have been dumped in there."
PC Thomas said that you never know what's in deep waters like this, no matter your swimming skills.
He added: "Farm irrigation reservoirs often have very powerful suction pumps installed to help with automatically watering the fields and no matter how strong a swimmer you think you are, you can still be sucked under and drown.
"The last thing we want is to be called out to pick up the pieces when it's gone badly wrong, and have to knock on a parents' door to tell them their child went out to play but will never be coming home."
If you see someone in trouble in water, dial 999 straight away and ask for the Fire and Rescue Service.