The man, Nelson Aguilar, 37, was riding one of the scooters with his 14-year-old daughter when it collided with another one carrying two of the daughter's teenage friends about 4 p.m.
The three teenagers managed to swim to a small, uninhabited island nearby, known as Goose Island, near Pelham Bay Park at the southern end of the river. The collision occurred near a railroad bridge in an area overlooked by the apartment buildings of Co-op City.
A New York Fire Department boat pulled Mr. Aguilar out of the water. He was treated by paramedics and taken to Jacobi Medical Center, where he was pronounced dead. The teenagers were also taken to Jacobi Medical Center, with non-life-threatening injuries.