Bites, venom and trailing prey
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 snake bites in the US, cottonmouths are seldom responsible for any deaths. In general they are not aggressive. However, perhaps 2008 was an 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. Therefore, its bite result in hemorrhae 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 let the rodent go, and trail it down afterwards.
Following a bite, all animals will show an immediate pupillar contraction that only last for a short period of time.
An effect observed on frogs in a study by Gennaro et al. (2007)2 was complete lung collapse. This definitely facilitate prey capture when the prey is amphibians such as frogs.
References
1. Chiszar, D. et al. Trailing Behavior in Cottonmouths. Journal of Herpteology 20(2) p.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)