1. never use the Lake Constance synonym "Swabian Sea". The people of Baden, who inhabit by far the longest part of the German shore, have a traditional rivalry with the Swabian Württembergers. And they don't speak "Swabian" either!
2. never get in the way of one of the ferries or excursion boats. You are driving a green ball, which means: I have right of way!
3. avoid contact with fishing nets. If your boat does get caught in a net: the initials on the attached canisters must be reported to the water police. You won't even risk your insurance cover, a downgrading or your excess - but you'll be in a lot of trouble if the skipper escapes.
4. don't believe everything you're served. The fish delicacy Kretzer, a type of perch, only comes fresh from the lake if the menu says "Lake Constance Kretzer". A "Kretzer from Lake Constance", on the other hand, has often travelled a long way from Poland or Estonia and is only prepared on site.
5. do not imitate dialects. Especially on the Swiss side, such language experiments are often perceived as an insult.

Editor YACHT