Vienna can get really hot in the summer - surprisingly hot - and good a/c can be hard to find. I thought I would put together a list of swimming spots for visitors who might want to see a different side of the city.
Bundesbad Alte Donau - This swimming spot is on the "Old Danube" a former bend in the river that is now a lake and huge recreational area. It is easily reachable by the U1 subway line with a stop called Alte Donau. The Bundesbad Alte Donau is a grassy, forested public swimming area with a pebbly beach that costs 8 Euro per adult for the entire day. It is shady, expansive and is a great place to chill for several hours. There is a small restaurant and lockers as well as showers and toilets. Lovely to sit in the grass and watch the small sailboats go by while reading a book.
Strandbad Gänsehäufel - This is an island in the Alte Donau, mentioned above. Reachable by bus from the U1 subway stop VIC. It has beaches, swimming pools, a ropes course, and lots of places to buy food. You can rent a locker for the day and take a working class Vienna vacation. It is also very woodsy and expansive. We love it.
Krapfenwaldbad - I mean just google search this place and look at the photos. The pool is situated on a hillside (mountain?) overlooking the city - what a view! Like all public pools in Vienna, it is cheap, but kept to a high standard.
Schönbrunner Bad - I bet you didn't know that there was a public swimming pool in the palace grounds of Schönbrunn. That's how we roll! This is a place to be seen and a very nice pool.
Free swimming platforms - There are free platforms all along the Alte Donau and the Donauinsel (Danube Island) where you can jump in the water and lay down a towel. There are some new platforms which have just been built (they look very nice) near the intersection of Fultonstrasse and An Der Oberen Alten Donau. If you are feeling adventerous, then consider using Next Bike (read this post - https://community.ricksteves.com/travel-forum/austria/biking-in-vienna-tips-from-a-local) to get to a platform. There are also popular platforms near the intersection of Kleine-Bucht-Strasse and An Der Unteren Alten Donau.
Rent a Boat - Probably my favorite thing to do ever in Vienna. Rent a small motorboat from Marina Hofbauer (right at the U1 stop Alte Donau) and swim in the Alte Donau from the boat. My favorite part is passing all of the little houses that look straight out of Denmark lining much of the Alte Donau. I also like to take a bottle of champagne and some sandwiches from Trzesniewski if I am feeling extra. Sunset is highly recommended.
The baths of Bad Vöslau - https://www.thermalbad-voeslau.at/. Want to take a short trip outside of Vienna to the spa town of Bad Vöslau? Easy to reach with a short train ride, the baths here feel like you've stepped into the 1920s. This is the real deal, folks. The pools are fed by a spring, which also happens to be one of the most popular brands of water here. You can bring bottles and fill up with water you would otherwise pay for in the store. But this is all about ambiance and beauty. You rent a small cabin, you have a nice lunch, you swim in the most pristine water you can imagine. It is affordable luxury, with not a tourist in sight (except Viennese tourists). I dare you do try tthis!