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Corey Griffin, 27, helped popularize the worldwide trend in which people either pledge $100 to ALS research or record themselves getting drenched by frigid water, then post the stunt online and challenge others to do the same.
Griffin jumped into the water from a building on a wharf that was a popular diving spot in Nantucket Island, Massachusetts, early Saturday morning around 2 am (0600 GMT).
Witnesses said Griffin plunged "into the water where he floated to the surface. Griffin then went under water again not resurfacing," the Nantucket Police Department said in a statement.
A nearby lifeguard was called to the scene to retrieve Griffin and emergency responders performed CPR, but he was pronounced dead at a hospital.
Griffin, who worked in finance, had been on the island to raise $100,000 for amyotrophic lateral sclerosis research.
A friend who was diagnosed with the the neurodegenerative disease inspired Griffin to help create the charitable challenge.
The Ice Bucket Challenge has become a global phenomenon with everyone from pop star Justin Bieber to former US president George W Bush and Microsoft founder Bill Gates drenching themselves in icy water.
And its popularity has spread around the globe, particularly to Britain, Australia, New Zealand, Canada and Germany.
Facebook said that between June 1 and August 17 more than 28 million people mentioned the challenge on the social network, and 2.4 million videos were posted.
As of Thursday, the ALS Association said it had collected $41.5 million in donations from July 29 to August 21 against $2.1 million in the same period last year.
Some 30,000 Americans have ALS, which attacks the nervous system and eventually leaves victims paralyzed.