MIAMI -- A Sunday afternoon at the beach ended at Ryder Trauma Center for an uncle and his 10-year-old nephew, NBC 6 reported.

The two were seriously injured when they were involved in a personal watercraft collision north of Key Biscayne at Bear Cut, police said.

The other watercraft left the scene, according to the U.S. Coast Guard.

The boy, who has a leg injury, was airlifted to a hospital.

His uncle, 19, suffered a broken leg.

The incident happened shortly after 2 p.m., investigators said. A good Samaritan called the Miami Beach Coast Guard station for help.

"The person who hit them just took off -- he didn't even bother to know if they were OK or if they needed help -- they left and left them," family friend Jennifer Molina said. "If it wasn't for that guy who went there, they would have probably been dead, because there was nobody there."

The incident is under investigation.