Because the Donau and Donau-insel are so long, you have many different ways to swim:
- swimming docks (alte donau) https://www.instagram.com/p/C_qMA8loXMv
- artificial beaches (CopaBeach)
- natural swimming (Neu Donau / Donau Insel) https://photos.app.goo.gl/d5iGqKP8uAjyYY81A
- Strandbad Angelibad for a roped off swim area with food and playgrounds
- Alte Hafenschenke where you swim on a tributary to the Donau
- swimming in a pool on a hill with a view of the city (Krapfenwaldlbad)
- swimming in a pool floating on the canal https://www.instagram.com/p/CiAb0gXufXx
- public pools cost only a few euros and have restaurants, slides, grass for picnics, sunbathing rooms (big rooms with no roof) https://www.heldenderfreizeit.com/wp-content/uploads/2020/07...
You've also got Sausalito-style restaurants on the water all along the river:
I normally don't like sweetwater for swimming and stayed away from the Danube for years. I was expecting the muddy smell and all, but this one's really great - I should've tried it earlier.
And enough water in the Wien to even contemplate it :D
That being said, Donauinsel is still quite close to the center and I personally prefer the relative quietness and greenery around there to swimming in the canal.
Those pontoon ferry boats travel over 30 km/h on the Kanal.
https://bsky.app/profile/jamestheswift.bsky.social/post/3l47...
According to their FAQ it is very much possible to swim in the canal under certains conditions that they mention as well.
Especially gravel beaches near Hainburg (which is technically not Bratislava) or on the other side between Devin and Karlova ves.