Security guards have been introduced at a reservoir in Sussex to stop unsupervised swimming.
Staff at Ardingly Reservoir said the cold water temperature, silt, as well as hidden pipework and machinery make conditions extremely dangerous.
Visitor numbers rose during lockdown, and people have been seen jumping into the water from raised platforms.
"People are risking their lives," said Chris Kinsman, director of Ardingly Activity Centre.
"I've never seen it anywhere close to being as busy as this before. They have no wetsuits, some come after hours, some people are drinking alcohol, it's a recipe for disaster."
Mr Kinsman was part of the search party when a boy drowned at the reservoir about 25 years ago, while swimming after a model boat.
He said people don not understand the potential dangers involved.
Submerged branches and sudden depth and temperature changes are among the hazards, as well as machinery hidden beneath the surface, which can start without warning and generate currents.
The activity centre, based at the West Sussex reservoir, hosts organised water sports and outdoor swimming training for triathletes.
Mr Kinsman said: "We follow very strict procedures, our instructors are trained in first aid, we have people out in canoes alongside swimmers, and the swimmers themselves have a buddy system and know the guidelines.
"If you're out there on your own then you don't have any of that."
South East Water, which owns the reservoir, said more than 50 swimmers, several boats and large groups of people were seen at the reservoir over the May Bank Holiday weekend.
A spokesman for the water company said the security guards will be on site to ensure visitors follow the rules and social distancing guidelines.