Not realistic, but a good yarn.
Too much womanising by Boxer, who was portrayed as some superhero.
Too many loose ends that were not explained or closed off, and way too many random coincidences. Naval boat commanders would probably roll their eyes at the tactics and actions of the Shark commander.
A good yarn, but that’s all. Not exactly a classic gripping adventure built around realism.
Then, maybe such a super sub is possible some day, who knows, with a superman like Boxer in command.