A snake! A snake! In a pond, 50 miles or inland from the coast in South Carolina USA.
And if the answer is “yes,” what kind?
Could be either a Northern Water Snake or a cottonmouth. It’s hard to tell from the picture, but a closer look at the head would do it.
Northern water snakes are definitely large, but non-venomous.
It could also be a Banded Water Snake. But the pattern makes me lean toward the Cottonmouth. Best to avoid in any case, obviously.
Another vote for cottonmouth: your picture looks like the second picture here.
See also about halfway down on this page for more about the cottonmouth and the northern water snake (and this piece of helpful advice)
Hard to tell by the photo. By the SC location, it could also be the Brown Water Snake, Nerodia taxispilota. They’re hard to tell apart if you can’t see the head clearly, and that’s not the best idea if it is a Cottonmouth.
UGA has a nice page comparing(scroll down) the Brown Water Snake and Cottonmouth/Water Moccasin, including some behavioural differences that might help out in your ID, Mr. Snake Hips. (I’ve always loved your username!)
As someone who usually encounters these critters weekly or more frequently, I’m not going very far out on a limb to state with 99.999% certainty that the critter in your picture is a cottonmouth.
Brown, Banded and Green water snakes all can look somewhat similar to him, but that dude is a cottonmouth.
In looking at the OP photo again, there is more white coloration on the underside of the head and neck, indicating a cottonmouth.
What I do think is quite cool snakewise is that the harmless Nerodia really mimics the toxic Agistrodon with body markings. Well, guess not so cool for people , and the snakes that get killed with a mis-ID.
Thanks, all. My suspicion was cottonmouth, also, based on something I’ve heard, but can’t attest to its reliability:
Water mocassins generally swim with most of their bodies afloat, and non-venomous water snakes generally swim with most of their bodies submerged.
Again, don’t know how reliable that is.
From the UGA link I gave above, that’s a habit of Cottonmouths:
Concur. The bands have a sort of pattern, two dark bands enclosing a medium band, with lighter bands separating each of these sets. Your photo matches that exactly (I don’t know if other species also match it exactly, but the other pictures in this thread do not seem to).
Also, if you look at the THIRD picture in Thudlow Boink’s cite, you’ll see the jawline of the moccasin/cottonmouth has a dramatic dark stripe with a pale line above it. I believe I can just barely see that same line in your photo, right at the waterline.
Coupled with the “swimming on top” behavior already noted (nice find) I feel some confidence it’s a moccasin. Or a cottonmouth. 
It’s clearly not a hip snake, sorry, Earl.
I am TIRED of these motherfucking SNAKES in this motherfucking THREAD!!!
What?
Regards,
Shodan
In my limited experience with water mocassins they have been what I would consider stout, that is, body is big around for thier length. Most water snakes I’ve encountered are fairly thin for thier length. It seems as if part of the body is below the waterline in the picture so hard to tell for sure but it does seem “stout”.
*and 3) often hold their position when disturbed, entering a defensive coil, or swim away on the surface of the water. *
Except when they swim TOWARD you, as one big assed cottonmouth did to me in my childhood. Michael Phelps would have been impressed.
Southern Trouser snake. Prefers moist to wet environments with some ground cover, doesn’t typically bite, but may spit venom if roughly handled.
Looks like a cottonmouth to me, too. And it’s the best match of username and OP I’ve seen in quite some time.