One of those watching the squid was a dive shop owner who leaped into the water to trail the creature. The footage above was caught by a submersible camera.
"This squid was not damaged and looked lively, spurting ink and trying to entangle his tentacles around me," the diver told CNN.
VIDEO: Giant Squid: King Of The Ocean
The animal stayed in the harbor for a few hours, before the diver guided it back out to the ocean. It's likely a juvenile, as it was on the small side for the species, at about 12 feet long (adults can reach some 30 to 40 feet long).
Experts told the network that it was rare to see giant squid - normally denizens of the deepest seas - swimming so close inland among boat moors. There was no immediately clear reason why this one had done so.