Police have resumed their search for a man who has been missing in the Georges River, in Sydney's south west, since yesterday afternoon.

The 28-year-old from Picnic Point was thrown into the water when his boat capsized near Revesby at about 4:00pm yesterday.

A 27-year-old Hurstville man was on board but he was rescued and taken to hospital.

An air and sea search was then launched for his friend, but emergency crews were unable to find any trace of him before dark.

Police say authorities have found the boat, but it was completely submerged.