Mocassins don't usually get more than 3 ft.
There is a VERY easy way to tell the difference between a napix and a true mocassin.
the napixs' tail (common water snake that sometime appear aggressive but are non-poisionus) extend SLOWLY from the body. Meant if the snake is longer than 3 ft, SKINNY, and has a long tail from the body, it's a water snake. If the snake is very fat and smaller, and has a RAPID tail "end off" (stubby butt sort of) it's the bad boy.
Mocassins also are usually found in the most nastiest swampiest (is that a word? lol) waters.

Either way though, yeah snakes are not fun.
There is some powder you can put down that messes with the jacobs organ. www.doityourselfpestcontrol.com