The British did lay a large minefield (as well as among other areas) almost the entire length of the east coast.
The North Sea has very dark water, therefore in RL raising the periscope whilst submerged would allow you to see perhaps a few feet beyond the bow of your sub.
Anything else IMO is simply unrealistic.
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Wise men speak because they have something to say; Fools because they have to say something.
Oh my God, not again!!
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