When I lived in Florida, a number of guys told me that they were hit by cotton mouths while wearing rubber boots and they were never punctured.
Lately I have been looking for rubber snake boots, they leave less scent. Cabelas had them, and I think BassPro has a model or two.
Now that I live in Texas, I have a lot of rattlers and cotton mouths to deal with, I love the rockies I have and my girl friend has. Remember, don't cinch the laces too tight, they will kill your foot above the instep.