Re: US Merchant Marines - Info on John Matthew Williams
What you are attempting is not the ordinary type of research. There are, undoubtedly, records somewhere. But where is a mystery. My guess would be the National Archives. There had to be records of all the ships because their cargoes were so important. A talley of all cargoes, delivered or lost, was undoubtedly kept because you can't fight a war without having tight controll of the logistics situation. And there had to be records of the men because each carried a rating of the job skills he could fill.
The merchant sailors organizations fought a long runningbattle with the US government for veteran's benefits for their members. They wouldn't have bothered if they did not have records of who did or did not go in harm's way.
Even tho' your father was a civilian the military POW records in the National Archives should have records of him because he was repatriated by the US Army like any other POW.
Sorry, this is so vague.
I would suggest that you get familiar with the web site for the National Archives. You can use their acronym, NARA. They are a big organization and there web site takes time to sort out but, if anyone can guide you to records it would be them. There is also web access to the modern Merchant Marine. They may have a historian or archivist who knows the whereabouts of what you are seeking.