Anglian Water has issued a warning after receiving reports of teenagers and young adults jumping into Pitsford Reservoir.
Social media posts have circulated showing people jumping into the water from the dam tower in a bid to cool off in the warmer weather.
Now the water company has teamed up with Northamptonshire Fire and Rescue to warn people to stay aware from the water.
Anglian Water’s Regan Harris said: “When the sun comes out we know the public, particularly young people, are more inclined to gather around waterways to socialise to enjoy the weather.
“Although our reservoirs might be a tempting place to take a dip to cool off as the weather warms up, there are many hidden dangers in them lurking below the surface.
“Swimming in our reservoirs is illegal, unless it is in a properly designated and supervised area.
“Reservoirs are there as storage so we can supply customers with drinking water.
“This means they are very deep and the water can be extremely cold in places – causing cold shock which takes your breath away and makes it hard to swim.
“There are strong currents in the water and hidden machinery lies just below the surface, not to mention the mud and weeds which can trap even the strongest of swimmers.”
The advice follows a series of instances over the last few years where two young people tragically lost their lives after swimming unsupervised in rivers and reservoirs in East Anglia.
One of those involved Huntingdon schoolboy Rony John, who died in 2014 during the first week of the summer holidays, while playing with friends in the Great River Ouse.
His death launched the poignant #RememberRony campaign across Cambridgeshire which raises awareness about the importance of water safety in rivers and open waters.
Jo Gouldson, safety officer from Northamptonshire Fire and Rescue Service, said: “Incidents involving water rescues do make up a proportion of the fire service’s callouts so we think it is really important to help spread key water safety messages as much as we possibly can.
“All too often people don’t take the appropriate care they should around water, whether that’s by jumping in reservoirs or on holiday in a resort swimming pool or even walking home along a river after a night out.
“Around 400 people drown each year in the UK, and drowning is the third highest cause of accidental death in children in the UK.
“Knowing how to stay safe in and around water is vital at any time of year.”