The coastguard had requested an immediate launch at around midnight as someone was thought to be in immediate danger in the sea.
The three-man crew used the lifeboat’s spotlight and the foghorn to search the area between the harbour and the Spa but were hampered by thick sea fret and low visibility.
However, they found the man, roughly 250 metres from the beach, at low tide.
An RNLI spokesman said: "He was wearing only boxer shorts and described as “severely hypothermic” by the lifeboat helm, who is a charge nurse at the hospital’s intensive-care unit.
“He was treading water and disoriented and starting to shut down. But he was conscious and breathing and managed to tell us his name”.
The crew hauled the casualty into the lifeboat and made an emergency landing on the beach, where paramedics, the coastguard, the police and the RNLI shore crew were waiting. He was taken to hospital.
Ten minutes later, the thick fog had lifted and by the time the lifeboat had been washed down and prepared for the next shout, the whole of the south bay was clear.
John Senior, lifeboat operations manager, said: “It was a difficult rescue because of the thick fog. The crew did well to find the casualty so quickly”.