The vessel used for shots of the PT-73 under way was a 72-foot type II Vosper MTB (Motor Torpedo Boat), a British design built under license in the U.S. for export to Russia. The war ended in August 1945 before the boat, the real number of which was PT-694, could be sent to the Soviet Union. The boat was then purchased by Howard Hughes and used as a chase boat for the one and only flight of his Spruce Goose aircraft. The boat was then sold to the studio - as there were few other real PT boats left in existence at the time - and some liberties were taken in reconfiguring it to look like a PT Boat. Vosper PT's did not have machine gun turrets on either side of the pilot house (though ironically the real PT-73, a Higgins design did) as the PT-73 in the show did. Other irregularities are the main mast aft and a small mast right in front of the cockpit. Shots of the crew aboard the PT-73 were filmed on a full-scale mock-up in a soundstage. "PT-73" was later sold to the mayor of Hawthorne, California, and was converted to a sport fishing boat. It was later destroyed when it broke loose of its mooring near Santa Barbara and washed up on the beach during a storm. The real PT-73 was a 78-foot Higgins boat assigned to Motor Torpedo Boat Squadron 13, which saw service in the Aleutians and in the Southwest Pacific theater. On 15 January, 1945 it ran aground, and was destroyed to prevent it falling into enemy hands.
You can read about Ernest Borgnine's 10 years in the real Navy here.