The Duke, so named for his constant youthful canine companion, was never portrayed as a Coast Guardsman. He played a Cold-War Air Force Colonel in "Jet Pilot," opposite Janet Leigh, a gruff Marine Corps Sergeant John Stryker in "Sands of Iwo Jima," as well as Major Kirby in "Flying Leathernecks," numerous Naval Officers and innumerable US Army (including Cavalry) officers.
John Wayne never served in the military, initially classified III-A (family deferment) and because of his age (34 at the time of Pearl Harbor). He was accepted into the OSS (Office of Strategic Services), but never got the letter, because it was sent to a former wife. This inability to serve was what led to his "superpatriot" status throughout his latter years.