Well most of one anyway. I didn't think it was a real one to begin with and just threw it in my trolley, but cleaning up in some rust remover and some vigorous brushing revealed "United States Property" and a serial number engraved on one side, and the letters G.H.D on the other which confirmed that it is the body of a real gun!
Definitely wasn't expecting to find this in the canal in Gloucester. I think it's from WW2 judging by the state of it and the markings, but I'm definitely not an expert.
Here's a picture of the other side, the G.H.D engraving isn't really visible on camera though:
Use a copper brass or bronze brush if possible. It'll do less damage to the underlying frame.