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Police said those aboard were watching fireworks at Cove Neck Shore. The National Weather Service said a thunderstorm moved through the area around 10:30 p.m., and winds never exceeded 10 to 15 mph.

A boater told Newsday he saw the yacht turn right and then tip over after it was hit by a wake.

“It was like in slow motion,” said Sammy Balasso of Oyster Bay. “All of a sudden, a lot of bodies were in the water.”

His nephew, 15-year-old Frankie Barbone, said the waves almost came up over the yacht.

Mr. Balasso said he put the spotlight of his 38-foot speedboat on the Silverton and then threw all the life jackets he had with him into the water. He said he rescued 20 people, who later were taken to the shore in police boats.

“Everybody was panicking,” Mr. Balasso said. “People were saying things like ‘Why?’”