Problem solved.
It was a matter of the skin I was using. That was the default GWX VIIC skin and I also tried one of fubar's Low res Skins but that showed the same blur for example at the ground plates. But with his high res all the sudden skin it looks really awesome. Clear and sharp textures no matter how close you get to the sub.
Now I have a new problem: I cannot decide which skin to use but that must be a common problem among the people who use fubar's uboat skins
It seems to be that his uploads are mostly low res skins though. Aren't there any high res versions ? In his skin thread all those screenshots looked more like high res especially the IX* ones.