I live in Australia, was born in Liverpool UK and stayed there until I was nearly sixteen, am now closer to eighty than seventy, and googled the big ship, to try to find its origins. I'm sure it is an historical reference to the sinking of a sailing ship, but no luck on the origins. The last verse we sang when I was a small child was, We all jump down to the bottom of the sea. Whichever vessel it was, the Atlantic, the alley between the Americas and Britain, would seem to be correct, the date we sang was the 19th of September.