Young man missing in Hawkesbury River

A massive water search is under way for a man feared drowned in a popular swimming spot north of Sydney.

The 21-year-old and his two mates were swimming at Windsor Beach, near Windsor, in the Hawkesbury River on Friday afternoon.

Friends have told police they tried to help the man after he began struggling.

But they weren't able to reach him and he disappeared underwater.

A search on Friday night failed to find the man.

Police divers were helping with the search on Saturday.