When swimming, what physical indication suggests a snake is a Northern Water Snake rather than a Cottonmouth?

The body rests mostly on the water surface.

A crucial behavioral difference useful for in-water identification involves how the two snakes manage buoyancy while swimming. The Northern Water Snake, being perfectly adapted to an aquatic lifestyle, tends to swim with a significant portion of its body submerged or resting low on the water's surface. In direct contrast, venomous pit vipers, such as the Cottonmouth, characteristically swim with the majority of their bulky body floating high above the waterline, often appearing as if they are riding on top of the water. Observing this difference in how the body is positioned relative to the surface can instantly resolve identification uncertainty, regardless of the snake’s terrestrial coloration.