From the Navy Lists of the time:
Arthur Roland Emmett
Commissioned 11 March 1940 as Temporary Lieutenant RNVR
Mar 1940 : HMS Watchful [Parent establishment, Great Yarmouth]
1 July 1940: HMS Lynx [naval establishment, Dover] “For Training”
4 April 1941: HMS Ronso (in command)
21 August 1943: HMS Sluga (in command)
15 March 1945: HMS Boscawen (naval establishment, Portland/Poole)
Ronso was a former Grimsby trawler, converted to a Minesweeper; she was based at Dover 1941-42
Sulga was a former Whaling ship, converted to a Minesweeper; I am not sure where she was based 1943-44
You mentioned Onetos and Oystermouth Castle – not shown, but may well have been when he was training