The cause of the woman's death was not yet known and an autopsy was planned, said Carmel police Chief Michael Johnson. He said she could have drowned or suffered some kind of medical emergency while wading with the girl in Lake Gleneida.
The lake, about 730 yards at its widest, is part of New York City's water supply system, and swimming there is banned.
The woman was identified as Pamela Kaner, 59, of Brewster. Police said she was caring for the girl while the girl's mother ran an errand.
The girl, whose name was not released, told police that Kaner brought her into the water in the early afternoon Monday and was holding her when something went wrong.
The child had no life vest on and was crying as she clung to the woman's body in the middle of the lake late Monday afternoon, said Carter Strickland, commissioner of the New York City Department of Environmental Protection. The agency owns the lake.
Two men and a woman in a rowboat heard the girl crying for help around 5 p.m. and found her holding on to Kaner's body, Strickland said.
The boaters pulled the girl from the water, brought her to shore and called police, officials said. She was treated at a hospital, but was not seriously harmed. Kaner's body was retrieved by firefighters, who paddled out in a commandeered boat.
No swimming or wading and no boats with engines are allowed on Lake Gleneida.
"She shouldn't have been in the lake," Johnson said.
The shore of the lake, which abuts the main road of the hamlet of Carmel, is littered with rowboats, most chained or cabled to trees. Johnson said the DEP grants permits for the boats. Signs on the shore say, "Recreation by permit. Entry for other purposes prohibited."
On Tuesday afternoon only ducks and gulls were on the water.
Kerry Browne of Carmel, a house renovator, said, "On a nice day like this, anybody would like to jump in the lake, but you know the rules. He said he hoped the girl would be able to recover from "holding on to a body like that."
Johnson said that in his experience, "It's very unusual when a drowning victim doesn't sink. The body usually goes to the bottom and pops back up when it decomposes."
Jeanne Buck of Patterson, who works in Carmel, said she saw a commotion Monday on the shore.
"I saw a woman carry a little girl up from the shore. I don't know but it looked like it was her mother."
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