In an exclusive interview with Eyewitness News, Bud Clarke said he knows someone witnessed another watercraft strike Andrew Weis' boat, and he's urging them to come forward.
"There were hundreds of thousands of eyeballs out there," Clarke said. "My objective is to say someone had to have seen something happening. And someone, I'm hoping, has the heart to say 'I saw another vessel hit him.'"
Weis, 48, of Gilgo Beach, was reported missing Saturday when he didn't return to the Anchorage Yacht Club in Lindenhurst.
His severely-damaged boat was found washed up off the Venetian Shores Park Sunday morning, and then a person on a sailboat found his body Monday morning nearby -- just off the shore of Tanner Park in Copiague.
Suffolk County police said there's evidence Weis' boat was involved in some type of accident, but they don't have any leads in the case.
Clarke said Weis' is an experienced boater and would have known how to handle a problem with the boat.
"On a level of 0 to 10, he's a 10 boater. The whole family is a boating family," Clarke said, insisting that another boater knows what happened to his relative.
"It does not make sense for him to have this situation happen if he was not hit by another vessel," Clear said. "Therefore, I am absolutely convinced that he was hit by another vessel. Absolutely convinced. And my objective is to let people know."
He added that he's not trying to undermine the police investigation.
"I'm not looking to second guess or outdo the police department, or any of the other investigating authorities - they know exactly what they're doing," he said.
Anyone with information is asked to contact the police at 631-852-6555 or Crime Stoppers at 1-800-220-TIPS.
Watch Kristin Thorne's full report at 5 p.m.