To be perfectly honest, I have never seen any mines. I have raided harbors, been up and down the east coast, and raid constantly in between ireland and britan. Im up to 1943 now, and although I look out for them as much as I can, especially when raiding harbors, I only ever see the nets. I use the observation scope under water to keep a look out in front of my boat.
I guess I have just been lucky, or maybe I just figured out that if you go right up the middle when entering harbors you never run into mines or nets?
Approaching from one of the sides always seems to involve hitting a net.