A speedboat driver would know he hit a person in the water, say police who are the hunting the skipper after a diver was struck and killed in Victoria’s Port Phillip Bay.

Investigators on Monday pleaded with the boat driver to come forward as they maintained a close watch of nearby boat ramps to try to find the suspect vessel.

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 a coast guard has 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.

After surfacing from his dive, the friend made contact with a skipper from a nearby boat to warn them that the two were diving in the area – unaware his mate had already been struck.

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.

The victim’s friend spoke with the skipper of a boat from the water before discovering his friend in distress, Senior Sergeant Nisbet said.

“He’d surfaced and he’d seen that [those on the boat] were close by to where they were swimming and he just wanted to make sure that they could see where they were.”

After finding his friend with “significant injures”, the diver hailed a nearby commercial boat. It ferried the pair to Frankston Pier, where attempts to revive the victim failed.

It was too early to tell if the suspect boat was speeding, Senior Sergeant Nisbet said.

There is a five knot limit (walking pace) in Port Phillip Bay within 100 metres of diving flags and within 200 metres of the water’s edge.

It was possible the skipper did not know they had struck a person.

“They probably do know now,” the officer said, noting significant media attention of the death.

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

Investigators shut down boat ramps in the bay following the accident, as they searched for the vessel.

“It’s your responsibility as a vessel operator to ensure that you keep a look out for everything and operate safely. It’s all responsibility back on the operator.”

Australian Volunteer Coast Guard spokesman Jeremy West said it was unlikely the skipper would have been unaware of the collision.

“It’s usually fairly obvious that you’ve hit something, particularly a solid object,” he told ABC radio.

“In this case, obviously, a person is quite solid. Hitting anything on the underside of a boat will typically give the noise to suggest that something has been struck.”

-AAP