During the ambulance ride, McRoy said, the man "was in bad shape, and greatly deteriorated." Parts of his body swelled up, the chief said, and he had trouble breathing.
The man spent the night in the Colleton Medical Center intensive care unit, Charleston CBS affiliate WCSC reported. The next day, he was flown by helicopter to a better-equipped facility, where he remained in critical condition.
The snake was captured by the injured man's friends, but authorities have not said what species it is.
Fatal snake bites are rare in the United States, according to the University of Florida's Department of Wildlife Ecology and Conservation, which said the "chances of dying from a venomous bite in the United States is nearly zero" because of access to high-quality medical care.
Between 7000 and 8000 people are bitten by snakes each year, according to the university, and about five to six Americans die of the bites each year.
David A. Steen, a research ecologist with the Georgia Sea Turtle Center, told the Washington Post that the story is an unusual one.
"It is unusual for rattlesnakes to climb trees. However, timber rattlesnakes live in South Carolina and they occasionally do climb, especially when they are young," Steen said in an email. "However, I have never heard of a rattlesnake in a tree over the water. And I have never heard of a rattlesnake in a tree over the water falling into a boat. I guess it is possible but is certainly not something that is particularly likely to happen."
Cottonmouths are aquatic snakes, Steen said, but the South Carolina snake most likely to plop into a boat is the brown water snake, which is harmless to humans. Those snakes "hang out in trees and fall into the water when they are startled."
Steen said that he did not know enough about the South Carolina incident to say what happened, but that people who get bitten by rattlesnakes often do so after trying to handle the reptiles.
