Good question!
Well, first things first: you can swim anywhere you find water! Seaside, lake, even local pools! So, basically, anywhere you go you can swim, Italy is literally surrounded by waters.
As for the water temperature, well it depends. I think Southern Italy is a lot warmer so the waters must be warmer as well. If you want to visit the parts of Italy "above Rome" (North, to be more precise), then it's best if you visit Italy in the summer: late July, August, early September. The temperatures in this period skyrocket! So the waters are usually warmer as well.