Your best bet to fill in those gaps is to try to obtain a copy of your father's complete merchant marine service record via the U.S. Coast Guard. His service record should identify the ships in which he served, applicable dates, training, shipboard positions held, etc. Please see this page from the Armed Guard website that I manage: http://armed-guard.com/searchmil.html. In particular see section A.2. Records of Individuals – Merchant Marine. You will have to contact the U.S. Coast Guard’s National Maritime Center in Martinsburg, West Virginia. The Coast Guard was and is responsible for issuing certain documents ("seaman's papers") to U.S. merchant mariners, so should have information about your father's merchant marine career. You will have to provide as much identifying information as possible about your father. There may be a fee for this service but I would expect the Coast Guard would not begin work without informing you of any charges.
Good luck. I hope this information is useful.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website