Lago di Saoseo is such a beautiful place in Val da Camp the South of Switzerland. The water is always very clear and calm. 
Keep in mind that the lake is only reachable by foot. 
The trail starts at Sfazù in between the Bernina pass and Poschiavo and is very well indicated.
The hike takes around 2-3 hours to the lake and the way down is easy too. 

There are post buses but only during high season in summer and in autumn, when the larches are turning into gold (golden summer) it is more beautiful to visit but you have to walk.