I understand where you're coming from on that. I'm generally not claustrophobic but I think that would trigger an attack with me too. I love the water, I swim every day but, in the past, I've almost drowned twice and it's one of my fears.
I've been on a Sub before and loved it! I'm in the process now of designing a WW2 Sub game. These new Subs are big and deadly. Get on a WW2 Sub sometime and you'll see how cramped they were! Doesn't bother me in the least!