Ambulance Victoria spokesman John Mullen said the man, in his 60s, was diving with a group when he had a rapid ascent.

"When they come up too quickly, these bubbles of nitrogen form in the blood and tissues," he said.

"It means they can't breathe properly.

"It can be potentially deadly if they're not treated correctly.

"They've got to be put into the decompression chamber."

The man was brought to shore at Wharf Street about 11am.

"They brought him in by boat ... and that's where they met up with the paramedics and that's where the helicopter landed," Mr Mullen said.

"They stabilised him and flew him across to The Alfred."

The man remains in a serious but stable condition.