This is indicative of a female. The tails of males would taper off much more gradually in shape towards the tip, as they need more room to store the hemipenes.
Cottonmouths are, naturally, venomous and get their name from the light-colored lining in the interior of the mouth. There are numerous species of watersnakes (genus Nerodia), which mimic cottonmouths, but are non-venomous. Below is a picture of a banded watersnake, N. fasciata.
Countless watersnakes are killed each year when people whom are less informed take the snakes' lives often to protect their family from a potential threat of a venomous snake. But...let's not kid ourselves, not everyone who kills a snake is rationally-minded. In fact, some kill snakes because it makes them feel "manly". Wow...please excuse my lack of enthusiasm for someone who is successful in killing something much smaller than themselves and perhaps because they're insecure about themselves.......
Anyway... Cottonmouths, as their specific name (piscivorous) implies, eat fish. Not soley, but they do consume fish and can consume fish as large as bream and small catfish. They also will prey on frogs, lizards, etc. Some can attain lengths of near 5' or so, but this is quite rare. Most are smaller, and individuals of 2-3' are more often encountered. Below is a juvenile snake I caught at the Zoo of Acadiana in Broussard a few years ago.