Ian, I have no experience researching the Merchant Marine, but it appears this web site may be of help to you. You will notice there is a link there, "How to get mariner's service record". http://www.usmm.org/index.html#anchor252856http://www.usmm.org/index.html#anchor252856
Also, the National Archives WWII POW records include listings for Merchant Marine prisoners. There I found listings for four Williams, but no John Matthew. The records I found available were for Williams were: Jack, Hoyt B., John F., and Lewis P.
You can check this info yourself at: http://aad.archives.gov/aad/http://aad.archives.gov/aad/ Follow the link: "Records of World War II Prisoners of War"