Police are investigating the circumstances surrounding a collision between a boat and a jet ski on Port Phillip Bay this evening.
Officers have been told a boat and jet ski collided on the water somewhere between St Kilda and Sandringham about 6pm.
The rider of the jet ski, a 16-year-old boy, suffered life-threatening head injuries.
He was brought to the St Kilda Marina by the crew of the boat and taken to hospital by ambulance.
The Marine investigation Unit is investigation and wish to speak to anyone who witnessed the collision.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential report at www.crimestoppersvic.com.au.