A seven-year-old boy is in a critical condition after being thrown ashore from an inflatable tube that was being towed behind a boat on Tasmania's Prosser River.

Key points: The boy was on an inflatable tube, also known as a "ski biscuit" when the 43-year-old man driving the boat lost control

The boy was on an inflatable tube, also known as a "ski biscuit" when the 43-year-old man driving the boat lost control Police said the man was shaken, and would be questioned as part of the investigation

Police said the man was shaken, and would be questioned as part of the investigation Waterskiing experts said many peopled did not realise how dangerous ski biscuits can be for the riders, and cautioned boaters to ensure they knew waters well before using them

Tasmania Police said a 43-year-old man lost control of the boat towing the "ski-biscuit" on the Prosser River Road about 2:00pm yesterday.

Paramedics attended the scene and treated the boy, who was then taken to the Royal Hobart Hospital by helicopter.

East Coast Division Inspector Peter Harriss said the 43 year old would be questioned as part of the investigation as soon as possible.

"He is understandably shaken by the incident," Inspector Harriss said.

"We would like to thank the medical staff, including Ambulance Tasmania paramedics and volunteers, who worked to stabilise the injured child before he was flown to hospital.

"He remains in a critical condition. Our thoughts are with him, his family and loved ones."

Police asked anyone who had seen a white boat at the time of the incident to come forward.

Ski-biscuits 'can be more dangerous than waterskiing'

Lance Shutt from the Nomads Water Ski Club told ABC Radio Hobart using a tube could be more dangerous than waterskiing.

"With waterskiing, the person who's being towed can let go of the tow bar, and have more control. [But] the person riding in the biscuit basically has got very limited control," he said.

"They can jump out of the biscuit, which, if you're going quickly, it's going to injure you when you jump out."

He said people did not know how "dangerous" the activity was until they were in that situation.

"The driver is the most important person, the person who's in control of the boat, for the safety of the person who's in the biscuit," he said.

"[The] driver should be asking what the local rules are, what local obstacles there may be that make it harder to do things safely, where the ski lanes are, direction and travel of boats."