It is believed foreigners and Thais are among the dead and missing. ''Three Thais and three foreigners are dead,'' one police officer said.
More could be still trapped in the ferry.
The sinking is the latest in a string of fatal incidents at sea off the Gulf holiday resort.
Detailed reports have you to come, but MCOT is saying that there have been six deaths and 15 serious injuries.
It's thought that the ferry, licenced to carry 150, was carrying 200 when it capsized, throwing passengers into the sea.
The ferry was one kilometre off shore when the vessel tilted. It was the last ferry of the day.
According to one report, a pump failed and the two-deck ferry began listing to one side.
Video taken on the shore showed cpr being administered to passengers at the dock. The vessel was returning from Lan island, a popular day-trip destination.
A series of speedboat crashes off Pattaya have left tourists from China and Korea dead in incidents attributed to the carelessness of the vessel's ''captains.''
Intensive seminars in marine safety have been held.
Phuket has had several close shaves with speedboats and the number of Chinese tourists who drown on day-trips is considered to be too high.