@Ruanek:
I think the rule is mainly to help out Germany. Perhaps it could be modified to allow subs out, because of the Kiel Canal?
Basically, you’re proposing that control of West Germany allows subs (though the canal could fit all contemporary German ships) transit between the Baltic and North sea. Which probably would be a fine house rule, but in most games it won’t ever come up as Germany has more fish to fry if they no longer control Denmark and rarely has so many subs in the baltic as cminke had.
The other problem with that logic is that if the Allies had control of Denmark, I find it suspect that the Kiel Canal would be safe as it is so near by and would be easily bombed (so maybe some subs could pass, but I think flotillas of them would be spotted and destroyed).
So essentially, if you control Denmark, you control the Danish Strait, and by virtue of being so close, you close off the Kiel Canal to anyone else. Which basically makes the Kiel Canal, if it were on the map, function exactly like the Danish Strait - control Denmark, control passage between the Baltic and North Sea. So in my opinion, adding in the Kiel Canal would be unnecessary as the Danish Strait accomplishes exactly that.
edited - the Bismark did transit through the canal