Revealing mistake: When the torpedo is fired at a Japanese ship, the underwater shot of the torpedo cruising reveals it being pulled by a long wire. It is also apparent that it was filmed in a tank with the reflection of the sun on the tank wall.
Revealing mistake: In this scene we see John Wayne talking to the deck gunner who is manning his post. There are also some sailors who are scanning the horizon for enemy shipping. It is obvious the background scenery of a cloudy night is a stage prop. You can also see a dark line coming down, meaning it was put together in panels. The fact that the clouds don't move or change shape also suggests the use of a stage prop. (01:14:20)
Revealing mistake: In this scene we see the underwater shot of the US submarine and the hull of the Japanese ship. It is clear this scene was shot in a tank as the suns reflection appears on the tank wall to the left of the shot.
Answer: A good submarine skipper ensure he always has a low periscope profile, as it's easily seen when out of the water too much. That's why he looks through the scope even while underwater, so the scope is only just out of the water.
stiiggy