You'll get more and better responses if you repost your question on the forum for New York City, which is more widely read than the Brooklyn forum:

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Brighton Beach is located at the heart of New York's Russian community, the largest Russian community outside the former USSR. It would definitely be an interesting visit as there's nothing else like it in the world. The boardwalk is a nice relaxing walk down to Coney Island and the original Nathan's Famous. You can eat relax with a cup of coffee in one of the numerous cafe's along the beach. However, the beach is certainly not the best in New York. Depending on when you go, it will be very crowded (stay away on Saturdays) and it's not the cleanest. But if you come on a weekday, it will be fine and the water is pretty warm for the north Atlantic this time of year.

Brighton Beach is more of an experience than a quiet beach. It is located in the heart of Little Odessa. You can pick up some food in a Russian Cafe, and eat on the boardwalk. Great People watching. From Brighton Beach, you can walk down the boardwalk to Coney Island and ride the Cyclone or the Wonder Wheel.

For swimming, my wife and I much prefer Riis Park, which is part of the National Seashore. This is not a trendy beach, but has a nice long strip of sand, and nice waves. The beach is fairly clean. As of yesterday, the sea was warm, but with the hurricane off the coast of the Carolinas, the sea was a bit choppy. Bring your own beach stuff.

For more pristine beaches, look to Fire Island or the Hamptons, both of which are easily accessible by railroad from NYC. You can make a day trip out of either.

We go by car, but there may also be public transportation available.