A mariner is a sailor. Finding details of what your ancestor did depends upon the year and the location. You need to provide more information for someone to help.
RootsWeb has a free mailing list for British Mariners to which this board is gatewayed. Information and subscription links can be found at
http://lists.rootsweb.ancestry.com/index/other/Occupations/B...The mailing list has an associated web page that might be of help:
http://www.mariners-l.co.uk/UKMasters.html