My guess is that your father-in-law was not aboard CITY OF FLINT. From records I found (see below) he was a merchant sailor at least as late as July 1943. CITY OF FLINT was sunk in January 1943. You say he was in the Armed Guard at the time of the incident he described to your wife but he could not have been in the Armed Guard until sometime in mid to late 1943 at the earliest.
The website American Merchant Marine at War has a list of names and ships of Armed Guard personnel killed and wounded during World War II; see http://www.usmm.org/armedguard.html. As a holder of the Purple Heart presumably he should be listed here, along with his ship. However the name of your father-in-law does not appear within this list, unless his name was misspelled. That said, one should not assume that this list is comprehensive.
A more definitive approach would be for your wife (as next of kin to her father) or another next of kin relative to request a copy of John Celentano's official military service record. (Next of kin = spouse, parent, child, sibling; persons not next of kin may be unable to obtain a complete service record but next of kin are able to obtain the full file.) See this page from the Armed Guard website: http://www.armed-guard.com/searchmil.html, in particular section II.A.1. - Records of Individuals, U.S. Military. You will need to contact the National Personnel Records Center in St. Louis, MO, providing as much identifying information as possible. There may be a fee involved but I don't know how much. You may make a request online or use a printed form (Form 180) and mail it. Be sure to request his complete military service file; otherwise you may receive only a copy of his discharge document.
Your father-in-law's service record should identify the ships to which he was assigned, injuries or illnesses, awards and decorations, etc., which would be useful in answering your questions.
Separately a next of kin can also obtain your father-in-law's merchant marine service record, via the U.S. Coast Guard. See the same web page noted above, at section II.A.2. - Records of Individuals - Merchant Marine. You would have to contact the Coast Guard's National Maritime Center in Martinsburg, West Virginia, again supplying identifying information.
Here's a little more information for you. The subscription website Ancestry.com (www.ancestry.com), primarily used for genealogical research, includes databases of the names of passengers and crewmen who arrived at certain U.S. ports of entry after a foreign voyage. Searching Ancestry.com I found a John Celentano listed twice. He was first listed as a merchant sailor aboard S.S. PERMIAN, a Panamanian-flagged tanker, on a voyage from New York to Halifax, Nova Scotia, and return in May-July 1943. (Many U.S. merchant mariners and Armed Guard served in Panamanian ships during World War II.) He was age 18, 5'5" tall, 156 lbs. and served as a galleyman (i.e., an assistant in the ship's galley). A year later he was listed in the Armed Guard aboard S.S. MISSIONARY RIDGE on a lengthy voyage (Baltimore-Philadelphia-New York-England-Panama-Marshall Islands-Pearl Harbor) in April-June 1944. He was a seaman first class (S 1/c), U.S. Navy Reserve, serial number (military ID number) 823-61-01. His serial number is an important identifying element in obtaining his service record.
My experience with Ancestry.com records is that there are many instances of misspelled names. (Ancestry.com records are based on copies of original records which often contain misspellings.) So it is very possible that a name like Celentano could be mangled in the original record(s) and be improperly indexed in Ancestry.com. So there may be more records but I couldn't find them searching on the correct spelling.
Incidentally, Ancestry.com would NOT contain a record of the incident that you describe. The original records on which Ancestry.com records are based were filed at the conclusion of a voyage. Since, obviously, the above incident involved the loss of a ship, the voyage was never concluded and the records never filed (and indeed were lost in the sinking).
I can confirm that there was a Liberty ship named S.S. JAMES BOWIE, built in Houston in 1942, which operated in the North Atlantic 1943-1944 and in the Pacific in 1945. She survived the war and was scrapped in 1971.
Good luck. Let us know of any success you may have.
Ron Carlson, Webmaster
Armed Guard / Merchant Marine website