It was a species of Nerodia. The specific image was of Nerodia rhombifer.
Now for the post Stacy wanted so badly!
Watersnakes vs. Cottonmouth
First, you should familiarize yourself with some basic terms in snake anatomy: head scales and body scales. With this knowledge, and the morphological knowledge I will give you about these two genera, you should have no trouble distinguishing the two. Once you’ve done that:
Nerodia species have highly keeled scales1 compared to the much milder keels2 present on A. piscivorous. A. piscivorous, in fact, has scales only slightly more keeled than A. contortrix.3 Nerodia species also have eyes which are plainly visible when viewed from above4 while the supraocular scales of A. piscivorous conceal the eyes.5 If you are feeling particularly brave, you can further verify an identification by looking at the rostral scale. In Nerodia, the rostral scale and internasal scales meet with a smooth, rounded appearance with the rostral scale highly domed, bulging anteriorly6 while the A. piscivorous rostral scale slants dorsally and anteriorly from the lip and meets the intranasal scales at a highly defined angle.7
Now that we’ve covered how they look different without even exploring coloration, I’ll now go into behavioral differences. A. piscivorous is commonly known as the “cottonmouth” for obvious reasons. The interior of the mouth is (usually) very white and as a defensive posture, they will gape and display this mouth for all to see.8 Nerodia, on the other hand, will usually not gape, and when they do, it will be obvious it is not A. piscivorous. Both Nerodia and A. piscivorous, however, WILL strike, and both are quite painful, with A. piscivorous being much more painful, but Nerodia hurting nonetheless. I can personally attest to the mind-numbing pain from a bite by A. piscivorous, one experience I hope to never again repeat. Nerodia bites are also painful, and should be avoided due to the risk of infection (they DO eat fish). While swimming, Nerodia species will keep the vast majority of their bodies below the waterline9 with only the tops of their heads exposed, while A. piscivorous will hold most of its head above the water.10
I know, this is the part everyone wanted to know, but to tell you the truth, coloration is far more difficult to use as a distinguishing feature since it can vary within populations so much. I had, in captivity, an individual A. piscivorous which appeared very light when I captured her, but while in captivity, she darkened almost becoming nearly black, only to again become lighter the following year. I distinctly remember this individual as she was the one that bit me. (Note: I feel this may be due to the change in diet of the A. piscivorous ) Nerodia coloration can similarly vary, only far more as various species exist in this genus. In any event, I do not like using coloration for genus identification, as so many patterns and colors must be covered. In light of this, I shall limit my discussion to A. piscivorous coloration. Generally, they will appear banded11 if their coloration is not almost totally black.12 Black individuals may have some sign of banding on the lateral portions12 of the bodies, but frequently, the parts which are seen (the back and top of the head) are entirely black. When viewed from the side, two light stripes can be seen above the eye on either side of the head above and below the eye.
This concludes this post about Nerodia and A. piscivorous distinction.