Its either a young black rat snake or a corn snake. Probably a black rat snake.

Hognose is the first thing that comes to mind for me too...they often spread their head and neck and hiss. A very cool snake...found my first two this year...if he was provoked any more, he would play dead, as they commonly do.

Cottonmouth / Water Moccasin (Agkistrodon piscivorus)

A lot of misinformation here that might be worth correcting: 1. Most people that are bitten by venomous snakes don't die. In fact, it is very rare for someone to die of snakebite in this country. The odds are under 1% of the bites. I personally know dozens of people who have been bitten by venomous snakes (I'm a herpetologist by training) and I know of one person who died. That person was in the mountains of Myanmar and was bitten by a highly venomous cobra and was unable to get medical help. If you are bitten in the US, you aren't going to die. Statistically, more people are killed by dogs, horses and falling furniture every year in the than by snakes. 2. I've never heard of a single person dying from an infection derived from a snakebite. I think you vet needs to stick to the facts that they know. 3. I've been bitten by many hundreds of non-venomous snakes(and one venomous) and have never had an infection from one any more than you would get from scratching yourself on a plant while gardening. A dog or cat bite is much more serious. 4. You should never rely on the lining of the mouth to identify cottonmouths. Yes, adult cottonmouths do have a white mouth lining, but young ones generally don't. Their mouth lining is pinkish, just like many other snakes. And if you are getting close enough to a potentially dangerous snake to see the mouth color, you are putting yourself at risk. Furthermore, cottonmouths don't always show their mouths when scared. Some do, most just try to get away. Cottonmouths are NOT aggressive. I know this because I have done research with the species in the swamps of South Carolina (where I encountered hundreds of individuals) and have also seem many hundreds of cottonmouths in Texas during the course of my birding and herping. Cottonmouths are slow and heavy bodied. They know they aren't fast so their only defense is to sit there and hope you don't notice them. If you get to close, they will sometimes flash their mouth lining but rarely strike, even if seriously provoked. If you want to know the truths about cottonmouths, there is a great article on Slate dispelling the myths and linking to the research - http://www.slate.com/articles/health_and_science/science/2014/09/cottonmouth_natural_history_myths_research_feeding_and_mating_habits_of.html As for identification of cottonmouths, they can be tough to tell from harmless snakes unless you know how to identify snakes properly. Things that DO NOT work for identification include: 1. Swimming on top of the water - sometimes cottonmouths to swim high in the water (floating on top). But they can (and do) swim underwater. And harmless snakes can swim above or below the water as well. So this "characteristic" sometimes works and sometimes is wrong. How useful an identifying character is that? 2. Have elliptical pupils (like a cat) - they do, but so do some harmless snakes. And elliptical pupils aren't always elliptical. They open up and become round in dim light, just like they do on your cat. So once again, not a safe characteristic to rely on. 3. Have triangular shaped heads - yes, they do...sort of. But many non-venomous species also triangular shaped heads or can flatten their heads out when threatened to make them look triangular. And by triangular heads on cottonmouths....do they mean seen from above or from the side? Again, not a useful character. It isn't hard to learn to identify the venomous species that live in your area, but you should learn the characters from reliable sources like field guides, not urban legends.