If you get off at the 1 train stop at South Ferry, you can actually see the carousel if you face the water with the buildings behind you (it's a short, round glass building). If you get off at Bowling Green, face the federal building (the Alexander Hamilton Custom House where the courts, the Smithsonian, and the National Archives are located) and walk around the federal building to the right. You'll immediately see Battery Park and the water. Follow the sidewalk along the park until you're behind the federal building. You'll see the South Ferry subway stop and nearby is the carousel.
It's in battery Park. 4 or 5 trains to bowling Green. 1 train to South ferry
If you google 'SeaGlass Carousel' in Google maps you should be able to find directions
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