Don't know a huge amount, but I'm pretty sure that a mariner could be a Merchant Navy seaman. I've got a Master Mariner in my family - he worked in the Merchant Navy, both coastal and foreign, so in ships around the UK coast and to other countries, presumably taking and bringing goods (unfortunately my ancestor doesn't describe which goods in his service record as a Master - but some do).
Usually Merchant Navy records specify date and place of birth too, so that you can hopefully confirm you have the correct person. My ancestor seems to have died at sea too, I haven't managed to pinpoint his death either! :-\
Good luck!