Having just googled "Bold Fisherman" and "African Queen", I came up with this link with the lyrics for the version of the Bold Fisherman in the film.

Totally different song, of course. I know of no version of the one you're playing which has the lyrics about his boat going "wibble wobble wibble wobble" ! I might consider learning it if there was!! I think the suggestion that this is why it has cropped up might be correct. I'm now thinking that I might try putting up my unaccompanied version of the song (a version which so far I haven't heard anybody else singing) just to see if I get the same reaction!