Paul Lawson, 16, reportedly got into difficulties while swimming with friends at Greenbooth Reservoir in Rochdale, Greater Manchester, on Monday.
Natalie Lawson said: "I begged him not to go. It is dangerous."
A 15-year-old boy also drowned while swimming at Ryders Hayes Mear in Walsall, West Midlands, on Tuesday.
Ms Lawson said she found out about Paul's death when she got messages asking if it was true about her son's drowning.
"I said, 'what do you mean is it true about Paul?'
"My head went miles away. It is just something a mother doesn't want to hear."
Paul's body was recovered by police divers. His death is not being treated as suspicious.
Ms Lawson is now urging people to learn from her son's death and not swim in the reservoir.
"It is not worth putting your family through what I am going through now and what his sister is going through," she said.
"Just think about it before you go in because you might come out this time but the second time, third time... you just never know when your luck is going to run out in these places."
One of his friends who was laying flowers at the reservoir said Paul was "a really nice lad, very well liked and funny".
He added: "It's awful."
As well as the death of the boy in Walsall, an 80-year-old woman was pulled from the sea at East Witterings, Sussex, on Monday.
Last year, 16-year-old Jack Pullen was pulled from the River Etherow, in Broadbottom, near Hyde, and 13-year-old Rafael Pizarro drowned in the Rochdale to Manchester canal in Middleton, Rochdale.
David Gillard, who lives nearby Greenbooth Reservoir, said the spot was popular with young people.
"We get a lot of teenagers coming up to cool off and what they tend to do is jump off the rocks," he said.
David Walker, from the Royal Society for the Prevention of Accidents, said: "Things to consider when open water swimming, even on a hot day, is that the water might be a lot colder or deeper than you were expecting and there may be strong currents and underwater debris that you cannot see from the bank.
"Consider how you are going to get out of the water before you get in, and be honest about your swimming ability."
Source: National Water Safety Forum
The mother of a teenager who drowned in a reservoir said she begged him not to go in the water.