I've been considering this for a while. Warning: My legal knowledge on the topic is quite limited.The waterways were a part of the commons.
I've never have heard the term commons ('Allmende') applied to waterways. In fact, that style of ownership died a slow death in most parts of Germany over the centuries. What's left is state-owned waterways, but I'm not sure how comprehensive that is. IIRC there were private concerns building and owning some of them, but I'm not sure how they fared in the century or so since. Drawing parallels, while most railway lines were unified in what TTL probably still is the Reichsbahn (I'd have to check older posts on the name...), there were OTL and probably TTL too some private ones remaining.
And then there is the fact that the central powers did better in WW1, leading to the question what happened to the important waterways: the Rhine and the Danube. OTL the Paris treaties made them international waterways, something that I can't see happening here to the same extent with no French border on the Rhine. That alone would inevitably change anything that happened in Germany afterwards.
However, even with AH still falling apart for some contrived reason, I'd expect a more concentrated effort on the Rhine-Main-Danube canal than OTL - the project was approved in the 1920s IIRC and only started building in the 1960s, finished in the early 90s.
Kiki remembered that that had been her mindset when she had bought the Meta, the rules had not even been a consideration for her. Fortunately, she hadn’t got caught on that odd trip up the Rhine to the Danube and Prague.
Did that one and the Danube-Oder-Elbe canal get built already?
Coming back to the first point, as ferdi254 said, regulation and licencing will not be nearly as hot topic an issue as in the US. Accessibility to licences might be more the topic, because most German would probably agree that river traffic has to be regulated, and that the equivalent of a car registration and driving licence can be expected as the minimum. (As annoying it can be to holiday-makers to not be able to just jump in)