There is a silly song sung locally here (Kent) by an amateur band called "Canal Folk" but probably from somewhere else (although if the Higham train tunnel which used to be a canal tunnel didn't have a hole in the middle it would be the longest chalk-lined tunnel in Europe) which comes in two versions. It is undoubtedly a narrowboat song.
The clean version of the chorus goes : -
"Boatie, boatie, /spit in the cut/Wiping his nose on a mopstick."
The other (which I favour as probably more authenic)goes
"Boatie, boatie, /shit in the cut/Wiping his arse on a mopstick."