Jacques, Kit, et alia:
The War Shipping Administration (WSA) took over many of the responsibilities of the U.S. Maritime Commission, itself created only in June 1936 under the Merchant Marine Act of that year, and that succession included the outright transfer to the WSA of the U.S. Maritime Service, for the training of officers in the Merchant Marine. Both the WSA and the Maritime Commission operated under the chairmanship of RAdm Emory S. Land, USN Ret., and during the war became all but indistinguishable from one other. Another tangible result of the Merchant Marine Act was the establishment of the U.S. Merchant Marine Academy at Kings Point, New York, in 1942-43. For a short history of the Maritime Commission and the wartime WSA, see
The need for naval gun crews was realized before the attack on Pearl Harbor, particularly to defend American merchant ships plying the Atlantic Ocean. The first Naval Armed Guard School was established at Little Creek, Virginia, in October 1941, whose graduates were sent for shipboard assignment to the also newly established Naval Armed Guard Center in Brooklyn, New York. Later, the Little Creek armed guard school was named Camp Shelton. Other Naval Armed Guard Schools were opened at Gulfport, Mississippi, and San Diego, California, and roughly speaking, operated in conjunction with the Naval Armed Guard Centers at New Orleans, Louisiana, and Treasure Island, California, respectively.
The armed guard consisted not only of gunners, but signalmen and radiomen, and even later, radar operators. A pharmacist mate or medical corpsman often rounded out the armed guard. The ordnance manned included the entire range from automatic rifles, machine guns, and light AA guns to sizable deck guns, usually 3-inch/50s, 4-inch/50s, or 5-inch/38s, but sometimes even larger pieces on the massive ex-luxury liners: 5-inch/51s, 6-inch, and 7-inch rifles once broadside guns on turn-of-the-century and World War I era battleships.
The first American merchant vessel provided an armed guard was very likely SS LARRANGA, which had been scheduled to take part in the dangerous run to Murmansk in the USSR in late November or early December 1941. She holds the distinction of being the first such armed American merchantman to fire on an enemy submarine during WWII.
One of the essential tasks of any armed guard was the instruction of the merchant crew in the operation of the deck ordnance. At the very least, merchant seamen were expected to be part of the ammunition train in serving these pieces, but even moreso, to take over the operation of the gun or guns when (highly) expected casualties occurred among the naval gunners. On some ships, only the largest deck gun was manned by the armed guard, with one or more smaller guns supervised by naval gunners mates, but otherwise manned by merchant gun crews (and see below).
This practice proved its worth in the instance of the famous battle between the American merchant vessel STEPHEN HOPKINS, on her maiden voyage, and the German raider STIER, off the south African coast in late September 1942. The German ship ordered HOPKINS to stop, but that injunction was ignored, and soon the raider found herself in the fight of her life. While the German ship used her five or six (see below) 15cm (5.9-inch) guns to great effect on the upperworks of HOPKINS, the American vessel concentrated the fire from her lone 4-inch/50 on her adversary's waterline and rudder. STIER was accompanied at the time by the supply vessel TANNENFELS, which raked the American ship with machine gun fire, killing several to many crewmen.
The officer commanding the armed guard, Lt. (j.g.) Kenneth M. Willett, USNR, was severely wounded in the abdomen early in the battle, but continued to direct the fire until losing consciousness and dying. As the naval gun crew fell, they were replaced by volunteers from the merchant crew. Soon, the gun mount was ringed with dead, dying, and wounded men. The last man to serve the gun was Merchant Marine Academy Engine Cadet Edwin J. O'Hara, who single-handedly pumped five more rounds into STIER before he too was killed. [The wartime protocol for academy midshipmen included substantial sea duty, where the midshipman held the rank of either deck cadet or engine cadet.] With her captain, Paul Buck, chief mate, Richard Moczkowski, and most of her gun crew all dead, and the ship in a state of wreckage, STEPHEN HOPKINS thereupon sank. Two hours later, STIER in a similar state, did likewise, with her survivors and previously taken prisoners of war rescued by TANNENFELS. For this action, Buck, Moczkowski, O'Hara, and three other merchant crewmen were posthumously awarded the Merchant Marine Distinguished Service Medal, with Lieut. Willett similarly awarded the Navy Cross.
This was not the first time that STIER had faced the fire from a USN armed guard: three months previously, off the coast of Brazil, she had encountered the new Socony-Vacuum tanker STANVAC CALCUTTA, whose stern 4-inch/50 was manned by the armed guard and the forward 3-inch/50 manned by the merchant crew. Both guns got off about 25 rounds each, one of them disabling a 15cm gun on STIER, before an ammunition magazine was hit by an incoming round, and assisted in the destruction of the tanker. Whether the damaged gun had been repaired before STIER faced STEPHEN HOPKINS, I do not know.
Two good general websites on the Naval Armed Guard are
and one specifically for the STEPHEN HOPKINS-STIER engagement is
The U.S. Army also provided ships' gun crews, in particular for its own transports (but some USATs had naval gun crews, so little is consistent or predictable). In the semi-independent Southwest Pacific, U.S. Army Forces in Australia (USAFIA) created the Ship and Gun Crew Command No. 1 to provide gun crews for the various small ships under its command, including 21 small freighters owned by KPM (Koninklijke Paketvaart Mastschappij...and I leave it to anyone to correct my Dutch spelling). American soldiers assigned to this duty received rigorous training in both naval gunnery and seamanship. As with the practice of their naval counterparts, radio operators, cooks, and medics were also provided. The work was highly dangerous and the men were almost always at sea. One assumes that the Dutch crewmen still aboard the 21 ex-KPM small ships were as well schooled in the operation of the guns placed aboard, mostly American in origin, but some likewise available British 3-inch or 12-pounder pieces, too.
Two U.S. Army "green books" are particularly relevant to army gun crews and other maritime personnel in the SWPac: Lida Mayo's 1968 THE ORDNANCE DEPARTMENT: ON BEACHHEAD AND BATTLEFRONT, and Joseph Bykofsky & Harold Larson's 1957 THE TRANSPORTATION CORPS: OPERATIONS OVERSEAS.
One other facet of information not necessarily relevant to your question: I recall that after the fall of the NEI, some Dutch marine officers were commissioned as officers in the U.S. Marine Corps. That is, they were NOT attached to or serving as liaison officers with the USMC, but for the war's duration wore USMC uniform and served as American marines, subject entirely to orders of that service. As I understand it, that stricture was (and still is) in conformance with the USMC's philosophy of complete oneness in its personnel.