"That's why the 'fine sandy bottom' is important because helps know were you are - the nature of the bottom was and is marked on the charts."
Quite right, which is one reason why this song, taken seriously, would be a danger to navigation. "55 fathoms (say 100 to 110 metres) with a fine sandy bottom" - where would that put you? Not on a line between Scilly and Ushant. Maybe on a lee shore (in a south-westerly) in the bight between Land's End and the Lizard, and God help you if you want to reach the Dodman. Any other offers?