His body was found near a water filtration plant at the Hosey Dam in Fort Wayne June 7.
31-year-old Hardy was reported missing by his mother on May 30.
The Allen County Coroner's Office said in a statement Wednesday that the cause of Hardy's death was asphyxia due to drowning and the manner of death was suicide.
"We take a look at the past history, both medical, including psych, and law enforcement contacts to give us a picture of the mental state of the individual", the coroner's chief investigator wrote in an email to WANE.
After a successful career at Elmhurst High School in Fort Wayne, Hardy played college football at Indiana. He led the Big Ten in receiving touchdowns from 2006-07 and recorded 2,740 yards with 36 receiving touchdowns before being selected by the Bills with the 41st overall pick. He also played for the Baltimore Ravens before being released in 2011.
In May 2014, Hardy was arrested after attacking three police officers. A judge ruled he wasn't mentally competent to stand trial, and he reportedly spent time in a mental health facility.