Victoria Police Marine Investigation Unit officers are investigating an incident where a jet ski overturned at Brighton Beach yesterday afternoon.
Investigators believe two men were on a jet ski opposite North Road about 3.15pm when their craft hit a wave and flipped.
The men were thrown into the water and struggled to stay afloat.
Witnesses called emergency services and police soon arrived.
Local police found one man on shore while the Air Wing located the other man in the water.
Officers boarded a boat with members of the public and pulled the unconscious man from the water and commenced CPR.
The 19-year-old from Narre Warren South was brought back to shore and taken to hospital in a critical condition.
The other man, a 20-year-old from Clarinda, was taken to hospital in a serious but stable condition.
Investigators wish to speak to anyone who witnessed the incident.
Anyone with information is urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppersvic.com.au