The men had headed upriver from Lake Oconee about noon when they noticed a fishing camp on the bank and saw an elderly man wearing overalls standing at the riverside.
The elderly Madison man raised a rifle and fired two shots, which hit the water in front of the boat, the men told a Greene County sheriff's deputy. The man fired a third shot, which hit one of the men in the boat - identified as Craig Barnett of Sycamore - under his left arm, according to a sheriff's department report.
After the shots rang out, the men sped away and headed toward an area known as Dyar's Pasture, where they found cell phone service and could call 911.
Mr. Barnett "showed me a dime-sized hole under his left arm and handed me a small-caliber bullet that he stated came out of the hole," a sheriff's deputy wrote in a report.
Mr. Barnett's wound wasn't life-threatening, authorities said.
The .22-caliber bullet was taken into evidence.
Sheriff Houston said the shore fisherman - identified as John Burke Yearwood, 86, of Madison - has put out fishing lines in the area for decades.
"He was just trying to warn (them) away from the fishing lines," Sheriff Houston said Tuesday.
Investigators found Mr. Yearwood at his home. Before he was arrested, Mr. Yearwood told investigators he had a rifle and had been at the river camp Saturday, according to the report.
Mr. Yearwood was booked into the Greene County Jail on an aggravated assault charge.
The state Department of Natural Resources helped with the investigation.