He said he was amused by the sighting, saying: "It's not a big one so I was amused more than anything else. He's floating around the 'Esperance'."
Harbour master Colin Bullock said: "A pond ornament has been disposed of, as simple as that.
"It's not real."
The foot-long ornament, which was just a head, had to be fished out with a net.
A RSPCA spokesperson said: "We haven't received any calls to our emergency hotline about a crocodile in the River Thames, however we have seen a video circulating on social media.
"It would be very unusual for a crocodile to be living in the River Thames and it's difficult to tell from the video if it's real or not.
"We would encourage anyone who spots the crocodile to contact the police in the first instance. We are happy to provide animal welfare advice, if necessary, if the animal does turn out to be real."