You can swim there if you want! there's a picture with ppl swimming in Viennas canal (Donaukanal) on the website and there's a whole association around it which claims 300+ members: https://schwimmvereindonaukanal.org/.
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.
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.