The woman, whose name was not reported, was believed to be pregnant, the Salisbury (N.C.) Post reported Saturday. Police said her son was "under 4."
North Carolina Wildlife Resource Sgt. Tony Sharum said the accident took place Saturday when a ski boat "traveling at a greater speed" hit the pontoon carrying the woman, her child, her husband and another adult.
Sharum said three or four of the pontoon passengers were knocked into the water, although those on the ski boat remained on board their vessel.
The child died en route to the hospital and the mother was missing in the water for approximately four hours until rescue divers found her in the Swearing Creek portion of the lake, the newspaper reported.
The father suffered minor injuries and the other adult -- whose relation to the family was not reported -- had serious injuries, Sharum said.
Sharum didn't say if the speed boat was traveling too fast, but he and the Davidson County district attorney's office will be reviewing the incident to determine if any charges will be filed.