The other girl was seriously injured in the accident Wednesday afternoon.
The dead girl was identified as Emily Fedorko, 16, who was about to start her junior year at Greenwich High School in the affluent New York City suburb. School officials said she was an honor student and a gymnast.
"This is a devastating loss for community," Superintendent of Schools William McKersie said. "She was loved by students and teachers."
McKersie said counselors would be available at the high school this week.
Two other girls were in the boat pulling Fedorko and her friend on a tube. Police said the boat's driver, a 16-year-old who was licensed to operate the vessel, turned when the two girls fell off the tube and struck them.
"It's a tragic sequence of events," said Greenwich Police Lt. Kraig Gray. "The girls were out having a good time."
The boat driver and a 15-year-old passenger pulled the two girls from the water and headed to Greenwich Yacht Club. Efforts to revive Fedorko were unsuccessful.
The death is the first boating fatality in Greenwich in more than 20 years, officials said.