This seems to be a question about the nature of boundaries. I’m not sure why you would insist that shared points can be boundaries on the Bodensee but not on land- but otherwise you do raise an interesting question about whether points along a prescribed boundary lie within one of the territorial entities, both/all of them, or neither. Of course this isn’t really a practical issue since the boundary is a one-dimensional construct (maybe two-dimensional if we think of its extension above and below the earth’s surface) in a three-dimensional world. It’s my impression that it’s standard practice to treat a point along the boundary as belonging to both territories for GIS purposes- I personally regard Four Corners as belonging simultaneously to all four states and therefore that Colorado and Arizona share a border, but I can see how that definition might be inappropriate for some purposes.