The geography does all the work here. The German shore is the northern one, so from Konstanz, Meersburg, Überlingen or Lindau you look south — across sixty kilometres of water to the Swiss and Austrian mountains. That means two things. First, from the shore you have backlight or side light essentially always, which is ideal for couples and demanding for groups. Second, the Alpine view depends on the weather and is most reliable in spring and late autumn, when the air is clear and the summer haze is absent.
The lake is also a mild-climate pocket, and that shifts the season: the fruit blossom on the Höri peninsula and around Meersburg starts in early April in a good year, a fortnight ahead of the rest of southern Germany, and in October the evenings are still warm enough for a session without a coat. What takes arranging is the Mainau — it belongs to a foundation, charges admission and permits commercial photography only by prior agreement. That is not a formality: there are staff on Mainau who ask.