Tenaya Lake is in Yosemite National Park, but it is not in the Yosemite Valley. The lake is up in the ranges at an elevation of 2,484 metres (8,150ft) (Wikipedia). It is one of the few lakes in the national park that is accessible by car (but only from June to November when the Tioga Pass road is open).
The waters of the lake are very clear, but are also very cold. I had a beautiful swim, but I only stayed in for about fifteen minutes. I slowly swam half way across the breadth, into deep but clear water, then hurriedly swam back to shore as the cold began to bite.
There was a keen wind blowing down from the peaks and gathering speed across the lake. By the time I was dry, I was probably as cold as I have ever been after a swim. 'Twas well worth it, though.