It is a capable swimmer, and like several species of snakes, is known to occasionally enter bays and estuaries and swim between barrier islands and the mainland.

[14] Many of the common names refer to the threat display, in which this species often stands its ground and gapes at an intruder, exposing the white lining of its mouth.

[18] However, a molecular (DNA) based study was published in 2014, applying phylogenetic theories (one implication being no subspecies are recognized), changing the long-standing taxonomy.

Adults commonly exceed 80 cm (31 in) in total length (including tail); females are typically smaller than males.

[27] Many would assume that the morphology of an aquatic snake should have a small, narrow head that tapers towards the back to minimize drag in the water, especially when capturing prey.

However, the pit vipers, and particularly Cottonmouths, display a contradicting structure, with its bulky, triangular head, which would be assumed to be poorly suited to water, yet it is not the case.

These crossbands, which usually have black edges, are sometimes broken along the dorsal midline to form a series of staggered halfbands on either side of the body.

These crossbands are visibly lighter in the center, almost matching the ground color, often contain irregular dark markings, and extend well down onto the ventral scales.

The watersnakes of the genus Nerodia are also similar in appearance, being thick-bodied with large heads, but they have round pupils, no loreal pit, a single anal plate, subcaudal scales that are divided throughout, and a distinctive overall color pattern.

[30] Bites from the cottonmouth are relatively frequent in the lower Mississippi River Valley and along the coast of the Gulf of Mexico, although fatalities are rare.

A few records exist of the species being found along the Rio Grande in Texas, but these are thought to represent disjunct populations, now possibly eradicated.

Its range also includes the Ohio River Valley as far north as southern Indiana, and it inhabits many barrier islands off the coasts of the states where it is found.

[38] The U.S. Navy (1991) describes it as inhabiting swamps, shallow lakes, and sluggish streams, but it is usually not found in swift, deep, cool water.

[31] Behler and King (1979) list its habitats as including lowland swamps, lakes, rivers, bayheads, sloughs, irrigation ditches, canals, rice fields, and small, clear, rocky, mountain streams.

[24] The snake is not limited to aquatic habitats, however, as Gloyd and Conant (1990) mentioned large specimens have been found more than a mile (1.6 km) from water.

[24] In tests designed to measure the various behavioral responses by wild specimens to encounters with people, 23 of 45 (51%) tried to escape, while 28 of 36 (78%) resorted to threat displays and other defensive tactics.

[40] When sufficiently stressed or threatened, this species engages in a characteristic threat display that includes vibrating its tail and throwing its head back with its mouth open to display the startlingly white interior,[41] often making a loud hiss while the neck and front part of the body are pulled into an S-shaped position.

[43] Other defensive responses can include flattening the body[42] and emitting a strong, pungent secretion from the anal glands located at the base of the tail.

[24] Brown (1973) considered their heavy muscular bodies to be a striking characteristic, stating this made it difficult to hold them for venom extraction owing to their strength.

Niell (1947, 1948) made observations in Georgia, and noted they were one of the last species to seek shelter, often being found active until the first heavy frosts.

They take advantage when bodies of water begin to dry up in the summer or early fall and gorge themselves on the resulting high concentrations of fish and tadpoles.

Heinrich and Studenroth (1996) reported an occasion in which an individual was seen feeding on the butchered remains of a feral hog (Sus scrofa) that had been thrown into Cypress Creek.

[8] Brimley (1944) described a captive specimen that ate copperheads (A. contortrix), as well as members of its own species, keeping its fangs embedded in its victims until they had been immobilized.

Agkistrodon piscivorus is preyed upon by snapping turtles (Chelydra serpentina), falcons, American alligators (Alligator mississippiensis), horned owls (Bubo virginianus), eagles, red-shouldered hawks (Buteo lineatus), loggerhead shrikes (Lanius ludovicianus), and large wading birds, such as herons, cranes, and egrets.

On the other hand, Neill (1947) reported captive kingsnakes (Lampropeltis getula) were loath to attack them, being successfully repelled with "body blows".

[8] Also called body-bridging, this is a specific defensive behavior against ophiophagous snakes, first observed in certain rattlesnake (Crotalus) species by Klauber (1927), that involves raising a section of the middle of the body above the ground to varying heights.

If weather conditions are favorable and food is readily available, growth is rapid and females may reproduce at less than three years of age and a total length of as little as 60 cm (24 in).

[8] Regarding A. p. piscivorus, an early account by Stejneger (1895) described a pair in the Berlin Zoological Garden that mated on January 21, 1873, after which eight neonates were discovered in the cage on July 16 of that year.

Wharten (1960, 1966) reported several cases where females found near their young stood their ground and considered these to be examples of guarding behavior.

[55] Constant persecution of the species and drainage of wetland habitat prior to development has taken a heavy toll on local populations.