The Cottonmouth snake is a so called pit-viper and it is capable of delivering a rather painful bite. Fatalities from Cottonmouth bites are rare, and from the average of six annual fatalities from venomous snakebites in the US, Cottonmouths are seldom responsible for any deaths. These snakes are, generally speaking, not aggressive.
However, 2008 is a considerable exception as an army trainee died from several cottonmouth bites during a training session in North Carolina.
The poison from Cottonmouth snakes is hemotoxic. A hemotoxic compound destroys blood cells, and prevents blood clotting. Its bite results in hemorrhage in those parts of the body where the venom penetrates.
Chiscar et al. (1986)1 showed that Cottonmouth snakes usually release adult rodents after striking them - even though other types of prey are most often held. After the strike, the cottonmouth would then release the rodent, only to trail it down at a later point.
Following a bite, all animals display an immediate papillary contraction that only last for a brief period of time.
An effect observed on frogs in a study by Gennaro et al. (2007)2 was complete lung collapse. This result is an inarguable effect that facilitates prey capture when the prey is amphibians, such as frogs.
1. Chiszar, D. et al. Trailing Behavior in Cottonmouths. Journal of Herpteology 20(2) pp. 269-272 (1986)
2. Gennaro, JF. et al. Neurotropic effects of venoms and other factors that promote prey acquisition. Journal of Experimental Zoology Part a Ecological Genetics and Physiology 307A(9) pp. 488-499 (2007)
3. Glaudas, X et al. Migration patterns in a population of cottonmouths (Agkistrodon piscivorus) inhabiting an isolated wetland. Journal of Zoology 271(2) pp. 119-124
4. Gibbons, JW. & Dorcas, ME. Snakes of the Southeast. (2005) University of Georgia Press