A CESSNA U206G FLOAT PLANE, N5328X, & A BELL 206A POLICE HELICOPTER, N4797R, COLLIDED IN FLT OVER BROOKLYN, NY. THE AIRCREW OF THE POLICE HELICOPTER WERE ON A TRAFFIC OBSERVATION FLT & HAD JUST REPORTED TRAFFIC CONDITIONS IN THE VICINITY OF A TOLL STATION WHEN THE COLLISION OCCURRED. THE FLOAT PLANE WAS ARRIVING ON AN AIR TAXI FLT TO THE WALL STREET AREA OF MANHATTAN, NY. A SURVIVING PASSENGER OF THE FLOAT PLANE SAID IT WAS IN A LEFT TURN WHEN SHE SAW THE HELICOPTER, JUST PRIOR TO THE MID-AIR COLLISION. THE HELICOPTER CRASHED THRU THE ROOF OF AN UNOCCUPIED GARAGE BUILDING. THE FLOAT PLANE MADE AN EMERGENCY DESCENT, WOBBLING SIDE-TO-SIDE, & CRASHED IN THE EAST RIVER. THE 2 REAR SEAT PASSENGERS EGRESSED BEFORE THE PLANE SANK. THE PLT & FRONT SEAT PASSENGER WERE INJURED & DROWNED. DIVER'S BELIEVED THAT NEITHER OF THE FRONT SEAT OCCUPANTS WERE USING THEIR SHOULDER HARNESSES. THE RIGHT FRONT SEAT PASSENGER'S SHOULDER HARNESS WAS FOUND STOWED.