65ft in the movie for the Dallas was periscope depth. I assume this is because the depth is to the bottom of the boat and not the top which makes sense. 50ft is a bit too shallow to go under container ships to take cover.

But what if those container ships have submarines deep up underneath them somehow. Perhaps the containers up top are just camouflage? Maybe the port being too busy to free up container space is just an excuse for ships to hang offshore.