There's a scene in Submarine where young Oliver Tate is moping on a beach, sad because he's in the middle of his first real break up, and he suddenly realizes that his ex-girlfriend is also on the beach. He walks up to her to apologize, but she's still angry. She tells him that he was always a dick to her. He says that he doesn't know why he was a dick, but he does know how deep the ocean is: it is six miles deep.
Submarine's tone is perfectly calibrated between being pretentious and heartfelt, between ridiculous and sad, and that exchange is a perfect encapsulation of that. Yes, what he said was the wrong thing to say - it's a total non-sequitur, and the fact that he was ignoring the emotion of what she was saying to recite a cold blooded fact is a good example of his dickishness. But it is also the most honest response he could have given, in a way. If he had apologized he would have been saying what he knew he was supposed to say, but instead he said what he wanted to say. And what he wanted to say is: I don't know what I'm doing. This is not a situation I understand. I want to go back to a world where the questions I have to deal with have right answers. Yes, telling someone who is hurt and angry that you don't have any explanation for what you did to them might be dickish, but that doesn't mean it isn't true.
Everything Oliver Tate does in this movie is wrong in that same vein. He ignore his girlfriend's emotional needs; he takes his parent's potential divorce personally even though it isn't about him; he bullies a girl at school to impress some other kids even though he knows it is wrong. But his choices are also perfectly wrong: some of the things his girlfriend asks of him are a lot to put on a person, particularly one who isn't fully mature; his parent's dysfunction is legitimately suffocating; and as heartbreaking as the bullying scene is, the fact that it also breaks Oliver's heart matters in a concrete way. It's fitting that he tries to get out of trouble by mentioning how deep the ocean is because he is very much a man out of his own depth.
When I wrote about Mean Girls recently I talked about how teen movies often have problem with building stakes because so many of the dilemmas that teens wrestle with seem a little overblown to adults who have a bit more perspective. Submarine shows the flipside of that coin, because it shows how some of the problems teens have can be worse than adult problems because teens have a more limited skill set to deal with serious problems. If we didn't know that Oliver was going to grow out of this phase of his life and make better decisions and build a better suit of armor to protect himself this movie would be thoroughly sad because it is about a kid getting kicked around by the world. Instead his melancholy can actually be funny, the way that an embarrassing memory can be funny if enough time has passed. Dan Savage is fond of telling teens "it gets better" and it's true; it does. (Although later it will get worse again, but for different reasons.) But no matter what, the ocean will always be six miles deep, and that's both comforting and terrifying.