It's my favorite lake for nude swimming in Vienna!
You can reach it by bus 92b, a 15-20 min walk from the stop Biberhaufenweg/Raffineriestraße or the best is by bike.
This lake is in a nice forest, the majority of the people going there are coming for nude swimming and sunbathing but on some days it gets very mixed with textiles. However, one shore is usually almost 100% nude, further away from the main walking route.
In the summer time the water temperature is very pleasant, you can meet many fishes, there are also some swans and ducks, really in the nature.
There are no facilities around, so bring your food/drinks with you and take the trash with you out of the forest.