When I make the Onion Valley to Whitney trip (which I have close to a dozen times over the years), I always take a swim in the upper Kearsarge Lake. I have been known to stay in for up to 10 minutes swimming around. Now this usually occurs after several quick in and out dips before my body acclimates to the temperature of the water.
I also have tried Guitar Lake on a couple occasions, but now I usually camp at the tarns above the lake so I end up bypassing the lake now.
I usually find that the greatest problem preventing me from swimming in the lakes along the routes is mental.
Paul