Das Boot is brilliant entertainment, but I really wouldn't put too much value on it as a historical document. IIRC there was even a group of U-bootwaffe sailors who wrote a book contradicting most of the events Buchheim claimed as true, and AFAIK many historians have refuted his views as well. So take Das Boot for what it is, i.e. one of the greatest war movies and / or books ever made, but that only
That said I really have to read / watch it again pretty soon, it's been a while already...