By CARRIE MELAGO, KERRY BURKE, and ROBERT F. MOORE
With additional reporting by Elva Ramirez

A depressed 14-year-old Manhattan boy plunged into the East River and apparently drowned in the frigid waters yesterday as his little sister watched helplessly, the frightened girl told cops.

"Please don't," the 6-year-old girl begged her brother, Sidney Hatchett, before he ran across South St. toward the river on the lower East Side. "Please don't."

But Sidney, who some relatives said had been depressed, ignored the pleas of his sister, identified only as Shakeemah by family friends.

When Sidney reached the promenade about 8:40 a.m., he took off his coat, climbed over the railing and plunged into the 34-degree water, Shakeemah told cops.

He surfaced once and then disappeared from sight, his sister said. "Then a shark ate him," she added, according to police sources.

Police believe the shark reference was likely the girl's way of communicating that the river had consumed her brother.

The boy's actions puzzled his uncle, 33-year-old Joshua Davis, who described Sidney as a "good, happy kid."

"The whole family's not doing too good. We can't understand why," he said. "We still hope he's alive somewhere."

Shakeemah, who had been walking to school with her brother, scooped up his blue-and-black jacket and ran home to Rutgers Slip, where she told her mother what had happened.

A police source said a surveillance camera in the apartment building shows the girl leaving with her brother and returning alone later with the coat.

The children's mother, Keisha Davis, raced to the scene and stared out across the murky river as cops began a frantic search for Sidney, looking in the water, surrounding blocks, the family's apartment building and his school, University Neighborhood High School on Monroe St.

NYPD divers spent about 10 to 15 minutes in the extremely cold river before being replaced by other members of the rescue team. Three U.S. Coast Guard vessels searched the river from Newton Creek to the Brooklyn Navy Yard.

Back on shore, a small group of cops pulled Sidney's mom away from the water's edge and comforted her as she stood crying, her daughter nearby.

The cops did not want either of them to witness Sidney's body being raised from the water, should he be found, a police source said.

By nightfall, he was still missing. Keisha Davis, a cashier at a nearby Pathmark, was too distraught to talk last night and Shakeemah was inconsolable, said family friend Monique Neville.

"She's holding up as well as she can," she said. "That was her big brother. She's going to need therapy."

Sidney was an excellent student and had even skipped a grade, according to Neville. He enjoyed using his computer and playing video games and sports, she said.

Several classmates said Sidney enjoyed school, but was routinely tormented by other children in his apartment complex. Neighbors agreed.

"Sidney was a smaller child," said neighbor Anna Rodriguez, 47. "The kids would pick on him because he was quiet."