Sub nets were used especially in WWI by capital ship to protect them from torpedoes. There was though a major set back: Travelling speed was down to 3-5 kts so the whole thing was not used in WWII in any way.

Loch Ewe is massively difficult to successful get into the harbor. Either you take your chance going down the west side (which is mined) and then go decks awash over the net, which has shore batteries on both sides; or you go the eastern route and have to go deck awash over 2 nets. Fortunately the nets are marked by buoys so you know where to surface, be carefull of the tugboats that are moored in various places, if they see you it'll set of the alarms. At least at Scapa you can stay submerged hugging the shore in order to get behind the subnets and into a firing position.