The body of the 28-year-old man was found in the waters off Gerritsen Beach just before 7:30 this morning.
The man was pronounced dead at the scene.
Investigators said his body had traumatic injuries, and an autopsy will be conducted today.
Detectives believe he is the brother of a younger man stabbed at a nearby soccer field off on Gerritsen Avenue.
The man, in his 20s, was found stabbed in the back at 12:45 a.m.
He was taken to Kings County Hospital, where he was being treated for his injuries.
Police said three suspects fled that scene.