I haven't been to this specific park, but nearby there is the Lobau forest (National Park) where there are anyway some lakes where you can swim and sunbath naked. It is very nice there, pure nature, there are no facilities (no toilet, no catering), but it is also free and surrounded with a nice forest.
However, there are some people going by and selling sometimes beer, coffee or some homemade cakes, and it is also an advantage that these places can be reached easy by bike, or even by bus from the metro station U2 Donaustadtbrücke (total about 30 minutes from the center of Vienna).
Also along the Danube river you can find many naturist swimming/sunbathing spots (south from Donaustadtbrücke)