Police are searching for the driver of a boat that struck a scuba diver in Port Phillip Bay on Sunday afternoon, killing the diver despite the efforts of paramedics to save him.

The diver, a man in his 20s, was hit by the boat between Frankston and Mornington shortly before 1:00pm.

A passing boat picked up the injured diver and his dive partner and conveyed them to Frankston Pier.

Paramedics were unable to save the diver. ( ABC News )

Paramedics performed CPR on the man but he died on the pier.

The second diver, also in his 20s, did not need medical assistance.

A police boat and helicopter have been deployed in the investigation.

Marine Investigation Unit detectives are investigating the diver's death and urging anyone who may have witnessed the incident or knows the identity of the driver of the boat involved to contact Crime Stoppers on 1800 333 000.