You MUST watch Das Boot. I started playing SH3 when my friend threw it out (THE FOOOOOL!!!), and I experienced my first naval sim, having been previously interested only by the air and land campaigns. Then I came here, started messing with mods, found GWX and it's riveting instruction manual (it was a manual???), and then came across many references to Das Boot.
Curiosity made me go out and rent the film (Directors Cut), and it just brought the whole submarine experience home to me, like I was actually there in some parts, and the game just reinforced that experience so much more after watching, it should be a pre-requisite of playing any SH game, at least SH3 anyway.
I remember going out on patrols and at certain times I'd think, 'Oh this is just like that bit in Das Boot'. Then after a while of thinking this a dozen or so times each patrol, I finally realised that at the moment I gun the engines in port to the moment I return home / sink into the abyss screaming 'MUMMY!!' (delete as appropriate), I was IN Das Boot.
Good enough recommendation?