The trouble with loitering near enemy ports I've found, and I've done this a lot for test purposes Liverpool included

, is that unexpected things do happen, collsions, ships that drift into walls and explode, convoys spawning nearby, unexpected visits by herr Goerings flyboys, that sort of thing. It IS a good way to see some of the interesting features of GWX that would otherwise go unnoticed though
I tend to stay away from them now, unless the weather out at sea is crummy and I'm in the vicinity anyway...
A slight caution, if you're ever tempted to visit st. Helena, they do have nets and mines too as I found out to my distress