South Wales Police shared the reminder after reports of young people swimming in the lakes at Cosmeston in Penarth.
Vale of Glamorgan Council had previously issued advice to help keep people safe.
A spokesperson for Vale of Glamorgan Council said: “Open water can be dangerous, even on hot days.
“Cold water shock, hidden hazards and strong currents can quickly turn serious.
“Please help us spread the message, especially to young people: don’t swim in open water, don’t jump or dive from height into water, only enter the water at lifeguarded beaches and designated bathing waters.
“Warning signs are in place and our teams will be out more over the summer, but we can’t be everywhere all the time.”
Visitors are also reminded that fires, barbecues and fishing are not allowed in the park.
They also warned that litter should be taken home to help keep the area clean, so everyone can enjoy the outdoors safely this summer.
