The 41-year-old Carrum Downs man was arrested on Monday after he contacted police following a plea for the skipper to come forward.

A 29-year-old Blackburn man, from South Korea, was skin diving with a mate on Sunday when struck and killed about 1pm in what the coast guard described as a "hit-and-run on the water".

The pair had been spear fishing several hundred metres offshore from Canadian Bay when the tragedy happened.

Skin diving is the sport of diving in deep sea water without a suit, sometimes while wearing breathing apparatus.

Water police Senior Sergeant Alistair Nisbet described both men as experienced divers who were following the rules.

"They were using buoys and floats and flags, everything that's required to obviously display that they are swimming out in the water," he said.

It was too early to tell if the suspect boat was speeding, Snr Sgt Nisbet added.

There is a is five knot limit in Port Phillip Bay within 100 metres of diving flags and within 200 metres of the waters edge, a speed the senior sergeant described as walking pace.

It was possible the skipper did not know he had struck a person, but likely he was aware he had struck something.

The accident happened offshore from Point Eliza, in the southern end of Port Phillip Bay.

The Carrum Downs man, who was arrested in Mordialloc, is being questioned by police.

Australian Associated Press