I have 3mm neoprene compression shorts (much harder to find, but out there) that I will wear under baggies when the water gets 56+ and air is 75+ because I just have an aversion to clothing at all and progressively shed the neoprene suits in descending thickness two steps ahead of normal temp chart guidelines as the water and air get warmer. I wear that combo with a 2mm topper (drylock XCEL is great) or a LS or SS nylon/lycra rash guard top if water is 62+ and air is anywhere from summery to warm. Cant wait til it's Africa hot and I go shirtless lathered in petroleum jelly and a 1mm neoprene loincloth.
Really though, all of that was true except the last one which I can't say won't happen.
My head doesn't get cold, nor does my core. My nuts, feet, and hands get cold in order from most to least. Shed the gloves at 56+, down to 1.5mm reef boots at 58+, then free balling at 66+. Til I realize I may not have kids if I keep that up.
I started using Body Glide a few years ago and haven't had a rash since. It goes on like deodorant. I just apply some on my inner thighs. http://www.bodyglide.com/products/anti-chafe/
Got a pair of quicksilver compression shorts that do the trick, keeps the boys in one place, prevents rash, and overall makes me feel snug as bug