They were fishing at Ladybower Reservoir in the Peak District when they were captured on camera edging close to an overflow which drains water away from the lake when it is full.
Flo Neilson, who was walking past when she spotted the anglers, told the PA news agency: “I thought it looked dangerously risky. It was amazing that they managed to control the ‘boats’ considering the force the water must have had.”
Ms Neilson said she saw them lingering near the edge for about a minute before moving away.
The overflow, one of two in the reservoir, is about 20 metres deep and takes excess water from the reservoir to the River Derwent and to local streams via a tunnel.
Severn Trent Water, which owns the reservoir, said it would be reminding anyone using boats on the water to stay safe.
A statement said: “We’re happy for people to use our reservoir for boating and fishing, and boats are available to hire from the fishery.
“However, we do ask that people stay well away from the plughole, as obviously this is really dangerous.
“We’ll be reminding everyone who uses the water to avoid the plug hole area and to stay safe.”