Staff Sgt. Nadya Lopez had been accused of failing to recognize or ignoring signs that 19-year-old Jason Tharp was too tired or incapable of continuing before he drowned during in a training pool on Feb. 8, 2005.
But the judge, Maj. Mark Griffith, acquitted Lopez in the nonjury court-martial just 40 minutes after defense attorneys rested without calling any witnesses. They contended prosecutors simply failed to prove their case.
"Sadly Marines do die in training," Lt. Col. Scott Jack told the judge in the defense's closing arguments. He added that Lopez "did nothing wrong. She was a professional water combat survival instructor."
Tharp's mother, Linda, cried as the verdict was read and told the judge, "I hope you see Jason every time you turn around and hear him screaming."
Several Marines testified that Tharp, of Sutton, W.Va., did not want to go in the pool and yelled loudly to get out during a survival floating exercise.
In closing arguments, prosecutor Capt. Doug Hatch told the judge Lopez was negligent for not getting Tharp out of the water.
The government did not contend Lopez was a bad swim instructor, Hatch said, but "she was negligent on this one day. She failed. She failed her fellow instructors in the pool, she failed the Marine Corps and she failed Jason Tharp."
Some prosecution witnesses had said Tuesday that they did not see any unsafe training methods by Lopez.
"He (Tharp) didn't seem necessarily scared of the water. It seemed like he really wasn't trying," said 1st Lt. Randy Brown, the safety officer at the pool that day.
If convicted, Lopez could have faced a dishonorable discharge, forfeiture of all pay and confinement for three years. About 20,000 recruits a year pass through Parris Island.