Yes, a water moccasin bite can kill a person, but it almost never happens. A review of U.S. snakebite deaths from 1989 to 2018 found only two fatalities attributed to cottonmouths, the other common name for water moccasins.
The study, published in the American Journal of Emergency Medicine in 2021, combined National Poison Data System records with cases found in medical journals and news reports. It identified 101 deaths from native U.S. snakes over those 30 years, even though the country sees roughly 5,000 to 10,000 snake envenomations every year. Rattlesnakes caused 74 of the 82 deaths in which the snake could be identified, about 90%, while copperheads caused five and an eastern coral snake caused one. The authors concluded that death after a bite from a native U.S. snake is unlikely, especially when the person gets medical care promptly. The risk was higher when people deliberately handled a snake or tried to kill it, or had been drinking or using drugs.
Image: Cottonmouth (Agkistrodon piscivorus), MH Herpetology, CC BY-SA 4.0, via Wikimedia Commons.