This is different from a submarine in some important ways. On a Submarine, you have many dozens of people to interact with. You are physically confined, but you have some social variety. Your mission is dangerous, but you have the comfort of knowing that it's been done before. Tours are, IIRC, 6 months long.
A Mars expedition is different. You're going to train with these people continuously for at least 5 years. During that time you'll constantly be on your best going-to-church-with-grandma behavior and never speak up about grievances because your actions are being monitored ad nauseum by a legion of shrinks. Then you finally get the mission go and you spend (optimistically) 3 months in a space the size of a minivan. Remember your last trip in a minivan? Imagine being trapped in it for 3 months with 5 other people while NASA is scheduling your day down to 15 minute increments.
If a thousand things you can't control happen to go right then you land on Mars. EVA suits on, and you finally escape that (obscenity laden) capsule. You see a horizon for the first time in what feels like forever. Then you work your ass off for a month and have to climb back in that (expletive) minivan for a risk-laden trip back home that takes even longer than the trip to get there. ... and during the entire trip you don't have a single shower.
It's not "exactly" like life on a submarine, is it?