If you spend a lot of time on the water in the Southeastern US, you definitely want to study up on these sometime aggressive boogers. True to their name, the inside of their mouth is a pale pink, almost white.
And the reason we know this is because these snakes are known to stand their ground, coiling up and threatening intruders with their open mouths, fangs front and center. Thus, similar to the rattlesnake, their common name was inspired by their natural warning mechanism.
Cottonmouths generally live around the water, in swamps, rivers, and on the edges of lakes. They can often be found sunning themselves on exposed roots and low branches.
The general coloration of these snakes is dark olive/black scales on top, pale scales on the belly. Younger cottonmouths have a more recognizable pattern to their scales, and a black mark above each nostril. However these marks fade over their lifetimes, giving way to a more solid, blackish color.