Hey Tim!
While I would never call Lake Michigan warm, the mid to late summer months are at least bearable. Prevailing winds tend to push all the warm surface water to my side of the lake. Racine is rather Shallow, too. The point I wanted to make to the gentleman asking the original question is... Wear what it takes to survive if you encounter disaster. We wear wetsuits even when the water is 72°. I am always very concious of water temperature. If I don't want to swim in it wearing a wet suit, I won't sail in it either. Simple rule for survival. I am sure there are some general guidelines for exposure available on the internet. I'll bet the Coast Guard would be happy to answer questions as well.
P.S. If I ever did sign up for the LOWISA, it would be based upon my possession ot drysuits for both me and crew.