All of the information seems to track with what I've found about the original song. Now I'm a bit confused as to Eddystone Light... So in some versions it starts with Rule Britannia/Mermaid version and others it has that chorus? In either case it would then appear that Married to a Mermaid was in place before that version of My Father was a keeper at Eddystone light (as there not being any mention of mermaids in the original version). So now I need to look up the history of Eddystone light?

The version in the Scottish Student's Songbook is not the pseudo-chanty. The version I got was in a Boosey songbook circa 1870. It doesn't seem like Dibdin lyrics to me - but as he wrote hundreds and I'm only familiar with about five, my opinion is far from expert!