At some point when my wife and I have more time (maybe, hopefully in a few years from now) I would love to spend a month or more along a lake somewhere in Europe, most likely Alpine Europe. Such a destination would ideally be beautiful, have somewhat warm water in summer, be public transportation friendly, and be near a fair amount of day trip type destinations, or even suitable for a few overnight trips as we would ideally spend the bulk of our time at such a summer home away from home, but could venture out a bit. Basically in this scenario it would be more like slow travel or even "summering" somewhere that just happens to be in Europe. It would also have a lot of nice walks, light hiking type opportunities as well.
Some places that I've been thinking about are Lucerne because it is so beautiful and seems pretty well located for a lot of day trips. Also, somewhere along Lake Constance might work, with its warm water and also good location. However, I personally don't find the area as scenic as some of the higher elevation lakes like the Vierwaldstattersee, or others like that. I have good memories of the Worthersee in Austria as well, but fear that it wouldn't be as strategically located. Locarno on Lago Maggiore is intriguing as it is in a beautiful area, somewhat accessible to other parts of Switzerland and yet part of the magical Italian lake zone.
Anyway, these are some ideas that are flowing through my head as I ponder "summering" by the lake in Europe.