[5] The common watersnake can grow up to 135 cm (4 ft 5 in) in total length (including tail).

[14] The two can be easily distinguished: the watersnake has a longer, more slender body and a flattened head the same width as the neck, round pupils, and no heat-sensing pits.

The cottonmouth has a fatter body, a wedge-shaped head with prominent venom glands that are wider than the neck, cat-like pupils, and heat-sensing pits between the eyes and the nostrils.

[14] Colubrid snakes also have flat scales on their heads, while vipers all possess smaller, rugose scutes.

The Lake Erie watersnake subspecies, N. s. insularum, was once endangered, but now benefits from the introduction of the round goby, an invasive species, which constitutes up to 90% of its diet.The common watersnake is common over most of its range and is frequently seen basking on stream banks, from which it dives into the water at the slightest disturbance.

Instead, the mother carries the eggs inside her body and gives birth to free-living young, each one 19–23 cm (7+1⁄2–9 in) long.

[29] N. sipedon has many predators, including birds, raccoons, opossums, foxes, snapping turtles, other snakes, and humans.

Its saliva contains a mild hemotoxic anticoagulant[citation needed], which can cause the bite to bleed more, but poses little risk to humans.

Juveniles typically inhabit lower-order streams adjacent to the larger-order waterways where adults are found.

This helps juveniles to avoid predators such as fish, birds, and turtles present in large water bodies.

[30] They tend to select areas further away from the bank out in the open in dead cattail clumps or low hanging branches.

The subspecies was first listed as threatened in 1999 after a decline due to eradication by humans, as well as habitat loss and degradation.

Endangered Species Act protections for the snake included designation of 120 ha (300 acres) of inland habitat and 18 km (11 mi) of shoreline for breeding grounds.