Oh yeah... 'das boot'... you know, wolfgan petersen had an entire, true-to-scale, 99% precise model of a type VII, in which the film was shot? The idea was to make the actors feel as cramped as the actual u-boat crews felt. And it shows! The model had special gearing to reproduce splashing and everything. THey could hardly fit the cameras in there. I've seen a 'making of' of this movie and the making of is as great as the movie itself.
And yeah, you're right. According to Mr. Wiki, again, an actual sub was only used for exterior shots.
"The
USS Redfish was used for many of the exterior pictures"