Hi Scott

Well, as we are very close to the Main here in Frankfurt I can only say that we do see quite a number of Kayaks from time to time.
As we are a relocation firm that assists others in transferring to the area I can only say that there are a number of Kayaking clubs here in Frankfurt directly maybe also a bit in Offenbach and Hanau. And this is basically is the core.

As there is commercial traffic yes, you do need to pay attention but I think kayaking up and down the Main or even towards the Rhine can be a great hobby.

You should also look into the dragon boat festivals which will take place again next year and really were quite an attraction.

If I can help you with finding the right kayak club or so please feel free to contact me.

Just by the way Frankfurt really has warmed up in the last days, the summer is just coming back.