I have done a few dives in Lake Mead. We dove during the summer months when the air temp was over 100 and the water temp at the surface was in the 80's. Wore a 3/2 wetsuit which worked fine until I got down about 30 or 40 feet. Then the water temp immediately dropped and I definitely felt cold. Visibility varied from almost zero to perhaps 40 or 50 feet. Went with Scuba Views dive shop which is located in Las Vegas at the corner of Rancho and Cheyenne. They did a good job.