Hi MJ,
I was looking over your GG Grandfathers ship drafts and the first on his list is HMS Impregnable which was a Training Ship where he would have attended his basic training into the Royal Navy, which would be square bashing, splicing wire tying knots etc. The next draft listed, being Vernon, was a training depot for underwater demolition. The reason I call it a depot is it was stationed shore-side i.e. a shore establishment. Mind you we're still in the 1880 so I don't mean all the underwater torpedoes, class A divers and acoustics of today also armaments, all would be nearer the birth of all these; and interestingly HMS Vernon in these days trained Royal Navy Divers. So that would be possibly were you GG Grandfather trained as a Clearance Diver.
I'm not doubly sure he did his diver training in 1880 but with him linked to Vernon it's more than probable he requested a divers course at a later date. In 1887 it looks to me he was an Acting Petty Officer so he could full-fill Coxswain Duties.
I do hope this information gives some real insight into the ships and ranks your GG Grandfather held. I will go thru' more ships he served and maybe find more interesting history.
Kind Regards......................Ron.