I think it would refer to Merchant Navy though its another word for sailor and
mariners are known as seaman. The National Archives at Kew have some Merchant Navy records on line, downloads cost £3.50 each.
I would go for Merchant Navy really is he was a seaman. He probably was in charge of cargo ships and freight ships as opposed to battleships and warships in which case he'd have been in the Royal Navy.
Thank you all very much for your help, The birth certificate simply says that my gg grandad was a seaman, allthough i have come across a relative whos occupation was a mariner according to 1841 census as it happens i have found out today that gg grandad was royal navy & not merchant.
info came from admiralty records found in greenwich naval school some of which are online at A2A national archives on line. For a small fee you can download the images if your lucky enough to find an ancestor. I have now got a full copy of my gg grandfathers naval history. It also gives date and place of birth, description i.e height,color of eyes and hair and complexion, plus a list of dates and ships served on rank and conduct.
My gg grandfathers last ship was HMS Victory 1889/1892 from 1892 to 1894 he is listed as shore patrol which would explain how he came to settle here in Portsmouth.
I personly found the A2A site confusing to navigate untill i got used to it and even now i still get lost in there.