Northern District Support's Inspector Darren Hopkins said a police vessel patrolled the river near Gravelly Beach for between three and four hours.

"We've got to reassess it now, it's been a week since he went missing," he said.

"If he was in the water we would have thought we might have found him by now.

"Just a reminder to the public to remain vigilant and report anything that looks suspicious along the river's edge."

Mr Moylan was last seen between 8pm and 9pm on January 25 rowing a small aluminium dinghy from the Gravelly Beach marina to his vessel, which is moored in the bay directly outside the facility. Mr Moylan lived on the boat.

The dinghy was found washed up on the shore about four kilometres north of the marina.

Mr Moylan, 42, is described as being 170 centimetres tall, of slim build with tanned skin and short, brown, wavy hair.

Anyone who knows Mr Moylan's whereabouts is asked to contact Beaconsfield Police Station on 63832002 or Tasmania Police on 131444.