I would like to visit Kreuzlingen, Konstanz and the Napoleonmuseum. As they are all located at Lake Constance and I'm living in Mainz I think it's not a bad idea to spend some time and travel by ship. Is this possible? Are there direct ferries or cruises from Mainz to the lake?
To make it short, no this is not possible. One reason for that are the Rhinefalls between Lace Constance and Basel. Ferries can not overcome these falls.
However, there are ships between Mainz and Basel, Basel to the Rhine falls, and ships from the Rhine falls to Lake Constance.
On Lake Constance itself, there are a lot of ferries.