I can't recall how many movies featuring subs I've got the opportunity to watch so far
, but I'd say those I won't forget are
Das Boot (for obvious reasons),
the enemy below (Curd Jürgens fanatic here), and
the hunt for Red October that will remain at the very top of my list (simply because it's the first sub-related movie I've been watching, and mainly because it features Sir Sean Connery in all his magnificence *joining hands in prayer*).