Where is it possible to swim in the wild near Bologna? I'm not looking for swimming pools but something like lakes or certain river spots.
I don't recall of any impressive lake near Bologna; however, if you are fine with driving a little further you could find some rivers and beautiful waterfalls near the border of Tuscany.
Here are few:
Cascata di Moraduccio, ~60KM from Bologna, located in Tuscany, right at the border with Emilia-Romagna
Cascate del Golfarone, ~100KM from Bologna, in Emilia-Romagna
Cascata dell’Acquacheta, ~105KM from Bologna.
You can find more suggestions in this blog.