The 36-year-old male's hand was spotted in the water as he struggled to stay afloat, on Thursday (May 25) morning near Blackfriars Road Bridge.
Tower RNLI crew launched into action, at 5.11am, at the request of the UK Coastguard after crew on a Thames Clipper passenger reported the incident and the man was pulled to safety.
They classed the rescue as a “life saved” (where, if it were not for the intervention of the RNLI, a person would most likely have died), saying he was just "five seconds away from drowning".
Craig Burn, helm of Tower RNLI lifeboat crew, said: "We reached the scene and located him in the water with just the top of his head visible. But as we approached his head went under and all we could see was an arm and hand as he was struggling to stay afloat.
"We were reaching out for him and his head came up again, and my crew grabbed his flailing arm and pulled him aboard the lifeboat.
"Based on how we found him, he was probably five seconds away from drowning. Given his temperature and the water he had ingested, if we hadn’t got him there and then, he’d have gone under.
"This just shows how important, how absolutely vital the advice of our Respect the Water campaign is. Water can be very dangerous and needs to be treated with respect."
The rescue coinicided with the charity's - Royal National Lifeboat Institution - launch of its national drowning prevention campaign - Respect the Water.
It aims to prevent people getting into distress on or near the water.
The latest fatality figures showed 42 people have accidentally lost their lives along the tidal reaches of the River Thames in the past five years.
Of these, an astonishing 40% did not even intend to enter the water.
During the morning rescue, crews had seen the casualty had floated with the fast-moving tide through one of the arches near Blackfriars rail bridge.
He was wearing dark clothes and had dark hair, which combined with limited light at that early hour, had reduced visibility.
The man was treated for cold and water inhalation and was taken back to the lifeboat station where he was handed into the care of the London Ambulance Service and the Metropolitan Polices Marine Police Unit.
In a key message in its campaign the charity informs how to deal with the potentially fatal effects of cold water shock.
It recommends one core survival action - floating - until the effects of cold water shock pass and you can catch your breath, then try to swim to safety or call for help.
RNLI is urging Londoners to remember the advice whethey on the River Thames or at sea after dats showed nearly three-quarters (74%) of people visit the coast at least once a year.
Guy Addington, RNLI Community Safety Partner for London, added: "We’re asking the public to remember this lifesaving advice, share with others and practice the survival skill of floating – it could be the difference between life and death.
‘"For those who are planning to go into the water, the best way to stay safe is to choose a lifeguarded beach and swim between the red and yellow flags, which is the area most closely monitored by the lifeguards."
If you see someone in danger in the water, don't attempt a rescue, call 999 and ask for the Coastguard.
The Respect the Water campaign will run throughout the summer.
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