COULD a North Queensland tropical island have its very own Loch Ness monster?

Some beachgoers at Magnetic Island, off Townsville, seem to think so after seeing a distinctive long, curved "neck" bobbing up and down off the coast on Friday.

But marine biologists who've seen the picture say the unidentified marine object is probably a piece of a tree or boat.

And a man who has dedicated his life studying mythical animals is also unconvinced it's a "Nessie".

The strange sea sighting has created quite a buzz on the island, with locals desperate to know: just what is lurking in the water?

One of them is marriage celebrant David "Crusty" Herron, who photographed it from a beach about 200 metres away.

"It was bobbing up and down in the water and at first I thought 'what's that'?" Mr Herron told AAP.

"Someone yelled out 'it looks like a Loch Ness monster'.

"I've never seen anything like it - it could be anything. We are all wanting to know what it is."

James Cook University biology professor Glen Chilton says while new and old creatures are constantly being discovered, even near the Great Barrier Reef, it's unlikely to be a strange aquatic beast.

"It's probably a piece of a tree or piece of a boat which has somehow broken away," he told AAP.

Australian cryptozoologist and self-proclaimed "yowie man" Rex Gilroy is keeping an open mind.

"It's hard to say from the photo," he said.

Mr Gilroy, who has authored books on mythical creatures, says he's aware of about 800 sightings of reptilian creatures with long necks and football shaped heads.

Some of those were from the Magnetic Island and Townsville area, he said.

The most recent was in October last year when a fisherman saw a creature with a large grey coloured body protruding from waters off Magnetic Island, he said.

While Mr Gilroy is keeping an open mind about Friday's sighting, he says it could very well be a dragon boat which sunk off Townsville last week.

Originally published as Has Loch Ness monster migrated to Nth Qld?