Mountain river or lakes are known to have ultra clear water as they mostly remain away from human pollution but Verzasca River in Switzerland is a notch more stunning with its clear turquoise water and vibrant colored rocks, as well as its treacherous currents. The actual 'Verzasca Dam' is a few kilometers up the river but if you desire some utter natural beauty, visit here. I've included some pics to justify the claims..
Verzasca river is a popular scuba diving location and the valley is used for bungee jumping. The diving conditions are usually best from late spring to early autumn.