I might look at the range map for cottonmouths:
http://en.wikipedia.org/wiki/Agkistrodon_piscivorus
It looks to be pretty accurate. When I lived in North Georgia I always had folks telling me about cottonmouths that they had seen. Well, cottonmouths do not live in that section of Georgia, it is too cold. We did have plenty of non poisonous water snakes though. Also, I have seen plenty of snakes that are terrestrial, swimming in the water. The way to tell the difference is that a terrestrial based snake will swim on top of the water, and a water snake swims under the water.
I do not believe that the Black Racer is endangered, unless things have dramatically changed lately. They are really pretty common.
You are right to feel that there are regional differences in the names of snakes. When I studied herpetology we were required to know all of the Latin as well as common names, for just that reason.
Finally, snakes can occur in "pockets". Right where I live we have mainly "bull snakes",Pituophis catenifer sayi, and I have never seen a rattler. However, the bull snakes have no problem with just coming into the house during the daytime in the fall. Two miles away, down in the Rio Grande Gorge, there are bunches of rattlers. 40 miles to the west there is a rattler under every bush in some places. Go figure. The books say that the rattlers do not come up to this elevation, but generally stay 3-500 feet lower. I just always wonder though, if the snake has read the book.