"David and I were racing back to the U.S., and they started shooting," she told "Good Morning America." I looked back, and I saw that David had been shot, and I turned around to go get him."
Hartley said she tried as hard as she could to pull her husband onto her own Jet Ski to take him to safety, "but he's a lot bigger than me.
"You can't imagine how awful it was not being able to help him," she said.
Knowing her own life was in jeopardy, Hartley said she was forced to abandon her husband. She took her Jet Ski at top speed back to the U.S. shore and placed a panicked 911 call.
U.S. authorities have searched Falcon Lake on the American side, to no avail.
The state of Texas had warned boaters and fisherman as long ago as April to stay away from the Mexican side of the lake. Since then, the drug wars along the border have gotten more violent and there have been reports of more pirate encounters.
Falcon Lake, part of the Rio Grande situated directly on the Texas-Mexico border, has recently become a haven for the pirates, and there have been at least five reported run-ins with pirates on the lake so far this year, although this is the first reported case of a death.
"The one thing I dreaded on Falcon Lake has happened," Texas' Zapata County Sheriff Sigifredo Gonzalez said. "The lake is not secure, the border is not secure because the incident that I dreaded the most has, in fact, happened. We cannot go to Mexico, we cannot recover that body, we cannot conduct an investigation, we have to tell the family we can't do anything about it."
The Associated Press as well as ABC News' Kevin Dolak and Sarah Netter contributed to this report.