I am German and I have been diving in Germany for more than twenty years.
As haeussf described, most diving in Germany takes part in small lakes and quarries. Usually the visibility is very bad. A visibility of 5m is "very good". Usually it is much worse and only very seldom it is better. Also the water temperture is quite cold, so even in summer many local divers prefer to use a dry suit.
For some better diving either you have to go to the far south or to the far north of Germany.
In the south there is Germanys biggest lake "Bodensee". It´s situated at the border between Germany, Austria and Switzerland. This is a very nice area in general. An d there are a lot of interesting divesites as well.
In the far north it is not the North Sea which is for diving. The German part of the North Sea is not a good place for diving. If you want to dive in the North Sea, better go the the Netherlands. It is very close by and there are a lot of interesting divesites.
The part in the north of Germany, which is realy interesting for divers is the Baltic Sea. In the Baltic Sea particullarly you will find a lot of wrecks, as well from the time of WW II as from different other periods.
If you have any particular questions, feel free to contact me.