Gavin Primo, a divisional police commander said the men were dressed in military uniform and took weapons stored at the police station.
Primo said were receiving treatment at the hospital in Bartica, which is located more than 128km southwest of Georgetown.
Witnesses told the AFP news agency that at least five civilians were ordered to lay face down on the ferry before they were shot and killed.
State television said police and soldiers were deployed to track down the attackers.
Sunday's incident came less than one month after 11 people were shot in their homes at Lusignan village east of the state capital Georgetown.
That incident was preceded by the shooting at the headquarters of the Guyana Police Force, resulting in two policemen being injured.