Just catching up on your excellent thread. Very interesting for those of us who have never been involved in offshore work. Interesting, although not really making me wish I was there.
I don't know the specific differences between boats and ships, I suppose Google has an answer. The one thing I do remember is that many years ago for reasons lost to history I had a trip in a Royal Navy minesweeper. It was not particularly big and it is possible I called it a boat. The officers on board explained to me that the Royal Navy only has ships. This was stated in terms that suggested I should commit the fact to memory. So I did.