Also have you looked at Passenger/Crews list of ships for his name?
Can you determine if he was a Merchant Marine or a Navy Armed Guard stationed on a Merchant ship?
How long was he a MM? Could he be found in the 1920 census with occupation as MM or listed on a census page taken on a ship?
There is a 1930 Merchant Marine Census, was he an MM then?
Is he in a "navy uniform" too with the others? Are you stating a "navy uniform" as in Navy Military Sailors orOfficers or do you mean 'naval uniform' as in Merchant Sailors or Officers?
What is his birth year...have you looked for a WWI Draft Registration Card...if he was a Merchant Seaman I do not think he would have been considered enlisted in the military and exempt from filing one if born between Sept 11, 1872 and Sept 12, 1900. Do ypu know where he was living 1917-1918?
Have you found him in the censuses from the first one he would have been in after birth to 1930?
Use www.familysearch.org for free or go to a Family History Center for help where to search and use genealogy websites and books of Ships passengers and crews (may be on microfilm too), or try your local library if you do not want to subscribe to websites like Ancestry or Fold3.
You may click on my user name for my email address and send me his name, date of birth and place born. I subscribe to Ancestry.com and be glad to do some looking on the internet for you. Put WWI MM in subject line so I know email is not spam.