According to the regional police force of Lower Austria, the teenager fell off his jet-ski while in a tight turn, and was hit by another jet-ski which was close behind him.
A witness rescued the young man from the water. The 13-year-old had suffered serious head injuries, despite wearing a helmet.
According to Austrian state broadcaster ORF, the victim had to be resuscitated at the scene by emergency services.
He was then flown by emergency helicopter to the regional hospital in St. Pölten.