Habitat: Commonly found in rivers, lakes, ponds, swamps, and cypress strands.

Nerodia cyclopion -- Mississippi Green Watersnake

Key Characters: Subocular scale between eye and supralabial scales; dark belly covered with pale half-moons; back scales strongly keeled; anal plate divided.
see Difference in photo of Yellowbelly at this site:

http://www.uga.edu/srelherp/jd/jdweb/Herps/species/USsnakes/Nereryfla.htm

Because they are found around water bodies, harmless watersnakes are often confused with the venomous cottonmouth (Agkistrodon piscivorus conanti). The cottonmouth has a triangular shaped head and vertical pupil. Cottonmouths can easily be distinguished from watersnakes. If the head is viewed from above, the eyes of cottonmouths cannot be seen while the eyes of watersnakes are visible; cottonmouths have elliptical pupils and watersnakes have round pupils; cottonmouths have a facial pit between the nostril and the eye, while watersnakes have none.

re: http://www.flmnh.ufl.edu/natsci/herpetology/fl-guide/Nerodiaeflavigaster.htm