Do you really not salute at sea?

In the U.S. Navy, one would render a salute only when "covered" -- that is, wearing a head covering. Since head coverings are removed indoors, salutes are not rendered inside compartments. On weather decks (out in the open), coverings are worn and salutes are rendered. The only exception to this rule is when the individual is under arms (i.e., wearing a sidearm or carrying a rifle), in which case the head covering is not removed indoors, and salutes would then be rendered. In the Kriegsmarine, the custom may differ.

This navy custom differs from the U.S. Army and Air Force, where salutes are rendered whether head covering is worn or not.

Most U.S. Navy and Coast Guard ships have what is called the "Nine O'Clock Rule." It is assumed that you have saluted all officers by 0900, so no more salutes need to be rendered.

In the Royal Navy, (at least during WWII) a man only saluted a senior rank if the latter was wearing his cap.It was the cap (symbol of rank) being saluted, not the man. In submarines, no saluting was done below deck, due to the confined space.

Give me five years and you will not recognise Germany again - Adolf Hitler

On top of the excellent replies already rendered, it should just be added that the entire interior scenes on the U-boat were complete fantasy. It would not have been brilliantly lit and spotless, with very little bric-a-brac lying about. Far from it. Watch Das Boot to see what it was really like. It was dark and gloomy, filled to overflowing with an unbelievable profusion of plumbing and valves and equipment and supplies everywhere, with no conveniences whatsoever. The captain most certainly did not have a huge cabin. Nobody wore spotless uniforms.

The men were dirty, messy, and smelly. No one had a chance to shave or shower for weeks at a time. The underwear they boarded with was the same pair of shorts they left with a month or more later. They could barely wash their hands, using salt water. Half the time it was sauna conditions in there, with guys walking around and laboring with no shirt, and often no undershirt. In the northern latitudes, on the other hand, it was frequently cold as a witch's, er, shoulder. Whether freezing cold or steaming hot, the air was fetid, mold grew everywhere, with condensation dripping from the ceiling and running down the walls.

The movie was absolutely great for its time. Conditions and conversation on the US surface ship are convincingly real. For conditions and conversations on the U-boat, they didn't have much realistic information in those days, and you have to take all that with a grain of salt.

Not too many people understand that the periscope on the U-boats was one deluxe feature they had. It wasn't a crude, spindly thing like they show in the movie. It came with an actual seat that the Captain could sit on, with power traverse.

With those caveats, the movie stands up remarkably well. It doesn't have much to come anywhere near its quality, made in those times or at any later time to date. The Key and The Cruel Sea and a couple of others dealing with the British Navy are right up there, and The Caine Mutiny comes close; that's about it.

Where the movie stands practically alone is in its pains to humanize both sides, and give you a reason to care about the men who served on both sides.