In sub-surface combat, there are only two things out there:
1. Other Submarines
&
2. Targets
That may be the case nowadays but that certainly wasn't so during WWII. Besides, I always find it rather interesting that that quote leaves out the third party: [EXPLETIVE] [REDACTED] [DELETED] [CENSORED] [DEAR GOD THINK OF THE CHILDREN] helicopters/MPAs.