Marion Lake warms up fast on a hot summer day. It's one alpine lake that I've actually swam in for a while, as opposed to just jumping in and immediately pulling myself out before I freeze. Be careful, though. It cools down quickly too.
Brock came back to this spot two weeks after we'd splashed around for whole minutes, wondering why in the hell it was so warm. He brought his girlfriend there, schlepping ten miles to take her swimming, but by the time they got there, whether because it was later in the summer, or later in the day, the water was frigid, just like any self-respecting lake in Grand Teton National Park should be.