The witness was also near the 13th hole when he saw the reptile grab hold of Cline and pull her toward the water.
"It dragged her in the water," the man told emergency dispatchers.
The Beaufort County Sheriff's Office later announced that the gator measured more than 9 feet long.
Seconds later, the man said the woman was "in the water right now" and that another person was out there with her. He then admitted that he did not know how to help.
"I mean, what can you do?" he said to the dispatcher. "I don't know."
Cline, a mother and kindergarten teacher, owned a home in the gated community, where she and her husband had planned to retire, her family told ABC News earlier this week.
The gator tried to attack Cline’s dog after she got too close to the water, and a tug-of-war for the dog's leash ensued between Cline and the reptile, according to officials.
Cline died at the scene after the gator dragged her into the water, according to David Lucas from the South Carolina Department of Natural Resources. Her dog was not injured in the attack.
The alligator was located and later euthanized, an official said.
In a statement on their Facebook page, Sea Pines Living wrote: “We are extremely saddened by this news and will share information with the community as it is made available."
Cline's death was the first documented fatal alligator attack in South Carolina in decades, according to local officials. The only other reported death that may have been caused by an alligator occurred in 2016 when a woman "wandered off from a nursing home" and was found dead with what appeared to be bite marks, Lucas said. There were no witnesses, so the circumstances surrounding her death are unclear.
ABC News' Bill Hutchinson, Rachel Katz and Emily Shapiro contributed to this report.