As part of our journey through Germany, we stayed for one week at beautiful Lake Constance (Bodensee. Disclosure, I grew up there, so I am biased). During this week we had daily temperatures exceeding 30 degrees Celcius. There is not much to do except taking a dip in the lake to cool down.
There are plenty of public bathing beaches around the lake, but to my knowledge none of them allows dogs. One needs to visit other places to cool down together. Below are the ones we used.
TIP: All these places are very shallow and stony. In particular near Strandbad Sipplingen seemed to have some very sharp stones. Watch out in case you want to throw something for your dog, and he jumps after. Also for the human swimmers, I recommend wearing some water sandals.
The only bath where the is also a kiosk directly at the bath where one can buy something to drink or icecream.