Plumb Beach is probably too small to be a neighborhood of its own, so when the Michael Sandy incident happened, the 'slug' under the news segment often read "Sheepshead Bay" or "Mill Basin".
Most accurately, the location should have been considered "Belt Parkway" or, most accurately "Gateway National Recreation Area." If the assault happened on the sandy area behind where the NYPD are standing, it actually would have occurred on Federal parkland. The median between the Belt Parkway and the park, as well as the parking lot are, as I understand it, under the jurisdiction of the NYC Parks Department. And while the city's Parks Department has an armed enforcement department, I think they receive a good deal of support from the NYPD in and around Plumb Beach.