September is generally characterized by nice and mild weather. Tuscans still go to the beach in September taking the last sunbaths of the season, especially on weekends, but we don't generally go swimming. September isn't too cold for a day on the beach. But both swimming or a day on the beach in September depends on the weather which is not predictable, and then on your personal preferences of what is cold or warm. In fact, if the weather is warm you can see lots of people swimming (generally visitors from colder parts of the world, such as northern Europe who consider the water warm ). But as I said, weather is not predictable and you'll have to watch weather forecasts when you'll be in Tuscany to plan a day at the beach.
The Gulf of Baratti is really beautiful. I personally love it and used to go there every summer when I was a child. The beach is nice and sandy while water is clear and calm. You will find both parking and free beach space.
If you are going to Baratti, I recommend you visit the beautiful Archeological Park of Baratti, where you can admire an Etruscan village. If you have time and you are interested in archeology, I suggest you also visit Populonia just few kilometers from Baratti, the remains of a stunning Etruscan city.
The trip from Castellina in Chianti to Baratti is about 2 hours and 15 minutes and around 130 km. I think it's doable on a day trip. Maybe a bit tiring but surely worth it.
I hope my answers help you in planning your day trip and don't hesitate to come back for any other questions. Please share your experiences afterward, it would help many others who are planning similar day trips
Thank you so much for the response, I think comments from someone that has experienced what we tourists are asking about is invaluable. I will certainly come back and blowout this board with my comments about my stay in Tuscany.. thanks again