On one occasion of which I have a recording Bob said in his introduction 'I'll try and make it rhyme'.

In the song he sang '... scared our girl', then after the next line
'She said "Blow this, I'm off ashore." '
added a spoken comment 'Doesn't rhyme, does it'.

From that same recording I can offer an alternative version of the mermaid verses.

'Then up jumps a mermaid covered in mud*
The skipper says: "I think we're on the Whittaker Spit"

Up jumps another one covered in slime
We took her down the foc's'le and had a good time'

*Another instance of an obvious substitution, but without suggesting the "F" word.