You are going to get a shock if you think the tunnel emerges anywhere near Denmark
I suggest fish anywhere, but do some research on licenses before you go. In Holland for instance, they are very keen on license checking, and penalties can be high. They also have some damn weird rules on what bait you can use and when. No livebaiting is allowed for instance, and a friend was fined for using worms at the wrong time of year.
Can be some good fishing though. My first dutch fish was a 20+carp, way back in about 1972. On the same trip my mate lost a fish that took a carp bait being retrieved. He didn't see the fish. I did. It was a pike that must have topped 30. He lost it after a good scrap, when it bit through the line.
Some good barbel too, well into double figures, but finding them in the huge rivers can be daunting.
Oh and if you do fish the huge rivers, watch out for the wash from barges that seem to be about 100 metres long.