During summer 23-30°C, and water about 22°C as well. Short or even shorts are enough in summer. Besides summer a wetsuit is needed. Surfing all year is possible.
Take the road N196 from Bonifacio to Sartene. Approx 5 km outside Bonifacio you will see a sign to Tonnara, approx. 1 km down the main road. Stop on top of hill to get a good overview over the beaches. Just before you reach Tonara, turn left and drive the small dirt road until you hit the beach of Stagniola.