Subject: RE: 68 deg water - no wetsuit
Did both races (1 sprint, 1 Oly) in 68 degree water this year, without a wet suit. The first one I was able to do a practice swim two days before and found out about Diver's Reflex. You can look it up on wikipedia, but basically, in cold water temps, the nose and face's receptors activate a mechanism for your body to slow the heart, vasoconstrict blood vessels, and conserve oxygen . During the practice swim, my adrenaline got me out 100 yards no problem, but then I couldn't breath any more and had to lay on my back since I thought I would drown. I couldn't figure it out since I am a pretty good swimmer.
When I got home, I looked up the symptoms and found out it was Divers reflex. What I did on both race days, without any problems were to get in the water early and acclimate my whole body to the temps. I also spent time just dipping my face in and blowing bubbles, all the while trying to relax through the divers reflex. I used wax ear plugs as well (though, I never use them training) since I read that cold water in the ears can also make things worse. Took a few practice "laps" and just concentrated on relaxing as the reflex went away. Anyways, that worked.
But once you get past the shock and start swimming, you'll be warmed up and it won't matter any longer. You will dominate on the T1 though, which is well worth it for a sprint.