I’ve been swimming a lot in the Neue Donau in September. It’s the most fabulous outdoor swimming pool you can imagine. As the weather gets colder, the water actually gets cleaner. There’s nothing like swimming out in the open air as far as the eye can see.
I purchased a half wetsuit on sale on a lark. It’s been great. I’ve worn it half a dozen times already. You can still swim in the Neue Donau in September but for sport swimming this year (more than 15 minutes) you need a wet suit most days.