swimming in the lake

Having decided it was easy, I went for a swim in the lake yesterday evening after supper. The wetsuit seemed to work well, at least I didn't feel my arm movement was restricted. It was really hard to start swimming in 66 degree water. I didn't feel cold, but I got tired easily and was particularly clumsy and unsteady when I got out. The lake is supposed to have warmed to about 76 degrees by the time of the triathlon. The other triathlon I have signed up for expects water temperatures in the low 60s, but it has a 1/4 mile swim instead of a half mile swim.