I was raising my periscope awhile back and caught a glimspe of something strange in the water ahead. It made me jump in my seat. I pounded Back Full and sure enough, my fears were correct. It was a tethered mine right in front of me and I just happened to get lucky in my up/down periscope searches for the nearby vessels I could hear on sonar.
However, I didn't see another on my ingress/egress of the port that time so, yeah, the density isn't too high.