Mr. Newman, I am a hobbyist who likes to lend a hand, if and when I can.
I checked the National WWII Memorial online database. As I understand it, this database is supposed to include the names of all WWII killed-in-action (as taken from either the American Battle Monuments Commission KIA database, or the National Archives KIA database, or both).
I could not find a listing for a John H. McNair, KIA, US Navy. There was one such entry, but his branch of service was listed as Army, and he was not killed during the war.
You said in your post, "who may have been killed in service aboard..." Could you expand upon what info you have? Are you sure he was killed, but not sure where? Are you not sure whether he was killed? Are you sure of his name and/or the spelling? Do you know where he was from?
If you can elaborate I (and perhaps others) will try to help you track it down.