Marine Investigation Unit detectives are investigating following the death of a diver in the bay earlier this afternoon.

The incident happened when a diver, a man aged in his 20s, was struck by a speed boat in the bay between Frankston and Mornington just before 1pm.

The injured diver and his dive partner were picked up out of the water by a passing boat and were conveyed to the Frankston pier.

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

The second diver, also aged in his mid-20s, did not require medical assistance.

Investigators are appealing to anyone who may have witnessed the incident or knows the identity of the driver of the boat involved in the incident to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report online at www.crimestoppersvic.com.au