The warning comes as temperatures are set to rise across the region, and people become more tempted to cool off in the water.
The water company has issued a plea to people visiting reservoirs such as Derwent, Grassholme, Fontburn and Kielder, asking them to stay out of danger by avoiding getting into the water.
Cold water shock can kill in less than a minute, and as well as being beauty spots the reservoirs are also operational sites with many hidden dangers underneath the water.
Paddle boards and dinghies are also not allowed to be used on the reservoirs - except where the company has an agreement in place with an accredited club that has safety procedures and personnel in place.
Tom Bell, Assistant Operations Manager at Northumbrian Water’s Waterside Parks, said: “We want everyone to enjoy our sites, but even more importantly we want everyone to be able to go home safely afterwards.
“Venturing into the water can have devastating consequences, not just for the individual but also for families, friends and the people involved.
“There have been many horrendous stories across the news recently of the heartbreak that cold water shock can cause, which is why we encourage our visitors to heed to the signs in place at our sites and please not enter the water.”
Sarah Litt, Community Safety Team Leader at County Durham and Darlington Fire and Rescue Service added: “We understand that going into open water can be tempting, especially when it is warm and sunny, but we are encouraging all members of the public not to take risks. Although the water looks inviting from the surface, it is still cold enough to induce Cold Water Shock, not to mention the dangers lurking beneath that you cannot see from the surface.
"If you don't have the what3words app on your phone, we would like to encourage you to download it. This means that if you were ever to come into difficulties either around the water or in a remote location, we would be able to pinpoint your location immediately.”
For more safety advice, visit – www.rlss.org.uk/Pages/Category/water-safety-information