Some of my Master's swimmers and I have been training for months to swim across Crater Lake. There is a boat manned by the park service that drops hikers off on Wizard Island and picks them up a few hours later. Instead of taking the boat back, we swam! About 6.3 miles, it took us around 2:40 of swimming time, but we were able to get out and warm ourselves on some rocks at around the halfway point. Water temperatures from about 56 - 61.
Crater Lake is the deepest in North America (we swam over one section of 1,700 feet deep!) and the clearest. You could look across and see other swimmers dozens of yards away as if they were floating in glass. It was AMAZING. I was so thrilled to be able to do this, especially after breaking my arm in June. I didn't have near the training time I'd hoped for, but it all held together during the swim.