Obese sailors???

Back then, the demand for men to man transports was so great that if you were a licensed Master in the Merchant Marine in peace time, then you would more than likely be called into Naval service (which happened quite frequently in both world wars) with very little concern for your physique. Officers in the Merchant Marine navigated cargo ships and were expert in both loading them and knowing the use of their cargo gear efficiently, so putting them in command of U.S. Navy transports was a no brainer.

I have seen it in photos. Once the war ended, he would have been canned due to the downsizing of armed forces and sent back to the Merchant Marine.

It's just a movie. My dad's archives have photos of Chester Nimitz in a Neptune ceremony, crossing the equator, and while not slim, he was an admiral, so he could get away with it.

I had a neighbor, who was also my art teacher in high school. That may sound odd, but he was the captain of an identical ship, a submarine tender, delivering food and fuel to subs in mid-sea, and he was probably only about 25 years old at the time. He probably got that because he had a college degree, and knew a bit about navigation from sailing on Lake Erie. He never looked like Burl Ives, and I doubt most captains ever did. He always said the two big problems he had were that the sailors were illiterate, and he spent most of his time teaching them to read, so they could find their way back to ship at port; he was also scared to death of a big red button in his cabin, that would blow the ship up if they were threatened.