Some of the ports are pretty easy to raid. Portsmouth on the other hand... yeesh! What a cluster that turned into.
I have to admit that Scapa Flow was a nightmare the first few times I went in - destroyers and armed trawlers all over the place up there. Took quite a beating on several occasions. Nowadays I know the patrol routes too well and don't bother raiding anymore.
Then there's Gibraltar. Just getting through the Strait is hellish, especially once the Allies start equipping their ships with radar. There's no way to escape them. I've only ventured into the harbor once and will probably not go back again.