In general, a master-at-arms is a sort of marine "policeman", but it is important to remember that the Titanic was a British merchant vesset and, as such, the duties of a master-at-arms may not necessarily have been the same as those which would pertain to the master-at-arms on a modern American vessel.
When I worked for English Heritage, "Archie", the Chief Custodian at Bishops Waltham Palace, near Southampton in Hampshire, had been a master-of-arms with Cunard. I can obly describe him as a sort of chief petty officer figure - the sort of person who would comment if one's hair was too long, or one's boots were not properly polished (mine of course always were!)