Despite numerous warnings about the dangers of swimming in these reservoirs, the company said hundreds of people have been caught taking a dip during the recent heatwave.
It comes after a man died in the River Ouse in York on Sunday and another man lost his life in the lake at Crookes Valley Park, in Sheffield, later that day.
The body of a man was pulled from the lake at Pugneys Country Park, Wakefield, on Monday, and the body of a 15-year-old boy was retrieved from a canal in Knottingley on Tuesday after he had apparently gone in with friends.
Yorkshire Water has brought in a security company to patrol a number of the 130 reservoirs it owns, including Digley Reservoir, near Holmfirth, and Redmires Reservoirs, in Sheffield.
Gaynor Craigie, head of property, said: “The last week has tragically seen four people lose their lives in open water in our region, following on from a further four fatalities earlier in the year.
“There are several risks associated with open water at reservoirs, which are often underestimated.
“These include cold water shock, unseen objects, the hazards of operational machinery and the underwater currents they cause. We would urge people not to underestimate these dangers and to speak to their children about the risks of entering open water.”
Yorkshire Water has also revealed 762 swimmers were seen at 14 of the water company’s reservoirs over seven days (from July 11 to 18).