They hardly freez up, but you might 
It’s possible to boat in Winter, but requires very good cold water cloths. Preferably a drysuit.
Things get easier beyond the Alps, even can have good water levels (snowmelt).
Easy floats on lager rivers are always possible (Rhein, Main, Elbe etc.)
Plan your route, see what water ways are there and get back for detailed Info.
S.