Navy (PROBLEM)

Can someone please explain to me HOW the hell navy actually works?? I (Ger) literally spammed out subs pre war and had like max mines in the two theaters into the Baltic See but they did nothing!! They didn't even put a single dent into the British fleet... When I play as the Brits, same thing the other way around, I just mostly still win because of the numbers.

So can anyone explain to me what I'm doing wrong??