The Camping del Mare campsite is in La Foce, on the left hand side of the beach in Marina di Campo.
Once you get off the ferry in Portoferraio, follow the signs for Procchio and then for Marina di Campo. After Procchio, at the junction for the airport, turn left and follow the signs for "La Foce - Marina di Campo". Keep going for about 1 Km, and you reach the entrance to the Campsite on the right.