This is a sand quarry, locally known as Písečňák or Pískovna Florian, where you can go for a swim. The surroundings are quite nice and it can be a peaceful spot.
If you are a guest at the local hotel, this could be a great private spot for you to relax and swim. There is also a nudist beach area available.
This is the important part: multiple visitors have reported that access to the water is exclusively for guests of the hotel. People who weren't staying at the hotel were turned away. In the past, when it was open to the public, visitors also mentioned that the entrance fee was quite high and that the water could get a bit green because the quarry is not very deep.