Police are investigating the circumstances surrounding the death of a 51-year-old Port Melbourne man after a marine incident at Port Melbourne Beach.

Investigators believe that the man was swimming in the water around Lagoon Pier at 5.50pm Friday evening.

Witnesses have told police that two men riding on a jet ski collided with the swimmer.

Emergency services were called to the scene and the 51-year-old was taken to The Alfred with severe head injuries.

The driver of the jet-ski, a 21-year-old Roxburgh Park man, has been assisting police and released pending further enquires.

Anyone who may have witnessed the incident, or have information is urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au

Senior Constable Adam West
Media Officer
VP18315/2012