There are years where I havent been in waters warmer than 64 f. , and I regularly dive in waters that are 38 f.
So its really a matter of geography, how good your drysuit underwear is, and how much lead you are able to load up beyond your perfect weight.
On that last point, let me just say that up here where we dive under the ice, we have a saying. lead is warmth That is to say, if I can add 10 pounds of weight without overwhelming the lift authority of either my drysuit or BCD, then I can add 10 pounds of air to my drysuit, allowing my underwear to remain fluffy and warm. The rules are just simply different where the conditions dictate the need for very good thermal protection. In warm waters, perfect weighting need not take into account the need for fluffy underwear.
Now onto a point of correction to a statement that was made very early in this thread.
Gidds:
There are LOTS of pretty things to see in cold water, they just aren't neon colored.
I beg to differ (and I'm not known for begging)! All you need to do is go diving in and around Vancouver Island. Dont take my word for it if you dont want to, just ask some guys like Uncle Pug or NW Grateful Diver. Theyll tell you, and even show you the pictures.