For Boro Park there is no need to included Brighton Beach in the mechitzos. For Flatbush, under the Boardwalk there are gates that can be used as the mechitzos (which also exclude the people on the beach in the tally).
According to Rav Moshe Brooklyn does not need to be enclosed by mechitzos that are within an area of 12×12 mil. Notwithstanding a reshus harabbim, mechitzos change the status of the area contained therein. Rav Moshe allows for mechitzos to be within 32 mil. The Brooklyn mechitzos are much less.