Hello Gaynor,
The records of Merchant Seamen at the National Archives are not online - maybe some day, we hope!
It's a case of going to Kew and searching through them, on microfilm.
I've been a couple of times and been successful, but I'm afraid at the moment, that's the only way.
If your ancestor was a Master Mariner, as Sue said, there should be a record. However, one of my ancestors said the same of himself on the censuses, and I'm afraid he must have promoted himself, because I've never found him!
Romilly, the same applies to your search - nothing online yet, but again, there are thousands of records at the NRA - here's another link that tells you "how to" - but it means a trip to Kew.
http://www.nationalarchives.gov.uk/catalogue/RdLeaflet.asp?sLeafletID=128#4Good luck,
Yorkslass