Firstly, querying the U.S. Coast Guard would be your first source for information about your father-in-law if he was a U.S. citizen sailing as a merchant seamen on U.S. flagged vessels. In 1952 he would have held U.S. Coast Guard-issued "Seaman" papers (or a U.S.C.G-issued "Officers License") to be able to be signed-on as a crewmember in any department (Deck, Engine, or Steward) on any Coast Guard regulated ship. As far as I know there is/was no distinction made as to whether the Semen's papers were for inland (lakes and rivers) employment or for deep-water (ocean-going) employment.
Since you are related to your father-in-law only through marriage, just how much information about your father-in-law the U.S. Coast Guard will give to you will be determined by the pertinent U.S. Privacy laws. Having an attorney to represent your interests to the Coast Guard might be necessary -- the attorney you hire doesn't have to be a maritime attorney (expensive!), but should be someone who has some experience or familiarity in appearing before U.S. government agencies.
Next, since the U.S. merchant shipping industry is highly unionized, your father-in law would also have had to be a member of one of the maritime labor unions representing merchant seamen. Therefore, your second line of inquiry would be to determine (if possible) what ship's department your father-in-law was employed in. With that information in hand you can query the appropriate union (or unions) which your father-in-law would have had to be a member of, given the ship's department in which he was employed.
Next, since you have evidently already contacted the National Archives regarding your father-in-laws military service, I would suggest that you contact them again, this time telling the docents that you are interested in information about your father-in-law's employment as a U.S. merchant seaman. I know for a fact that the National Archives has records of seaman's documents issued to seamen that they can show in lieu of being able to provide their actual passport. Because merchant vessels' masters habitually kept seamen's passports in their office safe during seamen's term of employment , this official U.S. in-lieu-of document was provided at no cost to seamen so they would have some way of demonstrating their citizenship to whoever, whenever, they were away from their ship. Those documents also include a passport-type photo of the seaman, if that's something that would also be of interest to you (as it was to me while doing my own family research there).
The docent's may also be able to help you find relevant merchant shipping records in the Archives, for example ships' crew lists if you know what ships your father-in-law sailed in.
And speaking of knowing what ships your father in law sailed in, he may have kept a few or all of his "ship discharge papers", which master's have been required for many, many, years to give to seamen who have completed the terms and conditions of their shipboard employment.
To summarize, I would advise you to contact the Masters, Mates, and Pilots union (the MM&P) headquarters in Linthicum, Maryland. The MM&P nominally represents vessel deck officers; if your father-in-law was a deck officer they will be able to help you further. But if not, I'm sure if you write to or speak with someone there they'll provide you with contact information for more appropriate maritime unions (such as the S.I.U., for example).
And then finally, keep up and further your contact with the National Archives in D.C. The docents there can be a great help in answering your questions and in your research on how your father-in-law died.
