The NC dept maps looks fairly accurate, but it cuts their range a little too much to the South. Water mocassins were and still are common in Fairfax County, VA, directly south of D.C., especially in and around the swampy man-made lakes. Saw one last summer at Burke Lake. Also saw many in the Occoquan River area. (I think this river may be the little black squiggle marked on the NC map, near the top of VA.)
Lived in Memphis for 28 years, and my God, when it rained heavily, they would come out from the creeks and onto the swampy ground of the public rifle range at Shelby Farms Park (2,000 acres, in the City limits, and yes, public shooting ranges.) They were thick, brown and very aggressive. The rangemaster would shoot them point blank with a handgun, primarily at the 100 yard mark, and drape their bodies over one of the target frames. It was a scarey thing to walk out in that mushy area to check your targets.
Mannyrock