San Giovanni is a small village across the inner bay from Portoferraio, three kilometres by road. Its claim to fame is the Terme San Giovanni, a modest thermal spa that uses local seabed mud rich in marine minerals — drawn from the bay just offshore — for traditional Italian fango treatments. The spa has been operating since the 1960s and is popular with Italian and German visitors for its rheumatic and dermatological treatments.
Day visitors can book individual mud and seawater treatments without a longer stay. The village itself is tiny — a small marina, a couple of restaurants, and a view back across the bay to Portoferraio's forts. Worth a half-day rather than a destination in itself; best combined with a meal at one of the harbourside restaurants.