"To our horror we spotted what appears to be some sort of large reptile," Ben told the News Shopper. "My colleague thought it might be a dinosaur but I corrected him that it looked more like a crocodile."
"Nobody saw Brexit coming so perhaps there is some sort of prehistoric Nessie-like creature living in the depths, or shallows, of Millwall Dock?" he said.
The photo set off a social media firestorm of users variously identifying the creature as a crocodile, alligator or an as-yet-unknown sea monster.
However, the Docklands Sailing & Watersports Center shed some light on the controversy by sharing a photo on Facebook of the "Dockness Monster" from another angle.
"We can confirm that it is not an alligator but a big bundle of ......ROPE!" the post said.
Judi Sanders, manager of Docklands Sailing & Watersports, said the water around the business is perfectly safe.
"This is quite funny although I am sure some people would be taken in by this and not want to come here," she told the Evening Standard. "It is rope, the water gets tested independently every other week."
The center's Facebook post offered a small consolation to Thames wildlife fans: "Sammy the Seal is still going strong."