Disney's version of "20,000 Leagues" got my vote but I also did enjoy "U-571".
A memorable one from 1978 is "Gray Lady Down" starring Chuck Heston and it's
now available on DVD.
yeah, Das Boot actually should be there. also i'm missing crimson tide, but i'd vote for Das Boot anyway. although other movies are more "entertaining" (and for this reason i'd rather watch these again), i believe the original version of Das Boot (not the shortened cinema version) portrays the real life on a submarine like no other.