There are a few guesses I would work on. If he married in 1916 as a "soldier" I would guess that he was one of those young men who volunteered for "King and Country" in 1914 or thereabouts. I believe that at the beginning of WW1 there was only volunteering, not calling-up but there was an enormous amount of jungoism and it was the "duty" of young men. Such things as the General Service Medal were, I think, handed out en masse after the war, so a lot of people did not actually receive them until well after they had left the army. Coming from a sea-faring area he could easily have chosen to go to sea simply as a job after he was discharged from his volunteering. If he was described as a "mariner" at death it could indicate he was unqualified crew, but it might be worth looking up Masters and Mates certificates just to see if he appears there. Otherwise I would start with the Register of Seamen and Shipping and theshipslist. Hope this helps.