Police say they don't know why the boat turned over during the Battle on the Bay race in the Great South Bay, a lagoon between Patchogue (PACH'-awg) and Fire Island.
Witnesses say some racing boats were doing more than 150 mph. They say the accident occurred far from shore and stopped the race.
Nearly 50 speedboats took part in the weekend-long event promoted by Great South Bay Racing.
The racing organization hasn't returned a telephone message seeking comment.