'All we could do was watch it drift out to sea'

A Victorian driver was loading a jetski onto his ute at a boat ramp when his vehicle reversed off a jetty and plummeted into the ocean.

The 27-year-old man was loading the jetski onto the trailer at the Mornington boat ramp at 3pm on Friday (AEDT).

Police Sergeant Kris Hamilton said the man swam back and forth to the jetty to save his personal belongings.

One onlooker, Mitchell Beer, said he was shocked by the incident.

"I was shocked really," Mr Beer told Fairfax radio.

"It was something you would only see in a car insurance ad. No one could do anything but watch it drift out to sea."

Nobody was injured as a result of the incident and drugs and alcohol were not a factor, police said.