A man has swum to shore after being thrown from a tinny in St Georges Basin, on the NSW south coast, while emergency services are using thermal imaging to search for his three missing friends.

The four men were believed to be on a fishing trip near Jervis Bay, when their boat capsized about 12.30am today, a statement from the Westpac Life Saver Rescue Helicopter said.

One man came to shore at Erowal Bay, and has been treated for hypothermia, while the boat was found on Wrights Beach, on the eastern side of the basin.

The helicopter crew is using the thermal imaging cameras to search for the three men, mostly in the basin's north-east.

The technology can detect body heat.

"As far as we're aware, they were fishing," the Westpac Life Saver Rescue Helicopter chief executive officer Stephen Leahy said.

"We don't know a great deal else. The man is being interviewed by police and there's an extensive search under way."

The helicopter crew have been searching for the men since sunrise in windy and choppy conditions, Mr Leahy said.

Water police and local Marine Rescue NSW boats are also involved in the search.

Yesterday, police divers found the body of an 81-year-old man after a dinghy capsized on Lake Macquarie, south of Newcastle.