Is the Kiel Canal effectively a sea zone for movement?
No. Hamburg and Kiel are a Port and a Naval Base, respectively, in the same area. When a Naval unit moves into either of them, it can immediately at no further movement cost appear at the other.
German naval units may move through the Kiel Canal counting the canal area (Hamburg) in their move.
Can naval units move from Bremen to Hamburg to Stettin, or the reverse, without entering the SE North Sea area?
No. Naval units cannot move from Bremen to Hamburg, or from Hamburg to Bremen, without entering the North Sea.
If the Kiel Canal makes Hamburg effectively a sea area for Germany then I think the answer is yes, as this would be land-sea-land and legal. If the canal is not effectively a sea zone then not, as land-land is not legal.
That's right. Bremen to Hamburg is land to land, and is illegal.