Well this thread is kinda old but I did encounter them in the english channel in OHII
I was dumb enough to select mare nostrum and i have to go around europe to get into the mediterranean lol
And as i was about to quit, i saved and said, hey lets go see if they work

They don't
I hit one and my boat just pushed it out of the way and it started drifting while rotating like it was a ball lol
It was really anticlimactic

MUYED
 

I found ... I have screenshots .......

grabs a shovel start throwing dirt back on the grave i do know the mines works in TWoS

Scapa Flow have many mines in the east entrance!