Mrs Pearce, took the photo on July 27 and reported her sighting to the Marine Conservation Society (MCS) where it was studied by experts.
Claire Fischer from the MCS said: 'Gill Pearce spotted the creature about 20 metres from the bay at Saltern Cove, near Goodrington.
'It was observed at about 15.30 on 27 July but by the time she had got her camera it had moved further out.
'She spotted it following a shoal of fish which beached themselves in Saltern Cove.
'The creature remained in the sea, then went out again and followed the shoal - this indicates it's not a turtle as they only eat jellyfish.
'We would love to know if other people have seen anything like this in the same area and can help clear up the mystery.'
Some people think the sea sighting could be linked to that of a sperm whale sighted off south Devon recently but Miss Fischer dismissed that explanation.
'They [sperm whales] wouldn't come that close inshore and the reptilian-like head counts that out - at least that's what the experts are saying.'
The sighting has caused a stir on the MCS website too, where theories range from sea serpent to salt water crocodile.
An MCS spokesman said: 'It was reported as a turtle as it had large front flippers and small back flippers and what appeared to be a shell but was also said to have a small head on a thin neck about two-feet long which craned above the surface like a Plesiosaur.
'No sea turtles do that with their heads and we do not know of similarly described freshwater turtles that grow so big.
'This is not a fake. The problem is the distance and clarity from which the photos were taken.
'The lady thought it may have been a turtle - but turtles don't chase fish.
'So at the moment it is "unidentified" - the person who reported it has trawled the internet and says the closest ID fit is a giant green sea turtle - but the description of the head doesn't add up.'
The organisation is now asking for people to keep a keen watch on the seas off South Devon and have appealed for more photos to be taken.
