If you hit Crater Lake during a real heat wave, try to save some time for a hike down to the lake and a swim -- bring your suit! (The trailhead is at Cleetwood Cove, I think.) Swimming in that indigo water is amazing -- the sun streams down all around you through the water, illuminating it in a bright blue-purple halo. The camera can't capture it (especially from shore), but this is our attempt at doing so:
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Click For Full-Size Image.
Just to emphasize the point, we have gone swimming in a lot of beautiful water -- in Hawaii and Costa Rica, in lakes and creeks in the Sierra, in La Jolla, etc. But this was far and away the most astonishing place to swim we've ever seen.
The water is very cold, but it is not unbearable -- maybe in the high 50s during the warmest part of the summer.