RMLI WR Swan HMS Laconia. HMS Hermes

My great grandfather WR Swan served in the RMLI throughout WW1 as a gunner on board what I believe were known as Q ships. He survived the war, but i would like to find acouple of things out about him. He was a member of the armed forces before the war, but I do not know what branch. I suspect it was possibly the Royal Marines as in 1 of his diaries that I possess he says that he was recalled at the beginning of hostilities. I would like to know his length of service, with whom and I would particularly like to know why he had a huge tattoo (clearly visible in photos) of a clock face showing twenty to five across his chest. Can anyone out there help please ?