Just pay for the ferry. It's $2. The hammock is free.
As David mentioned, once you're in the island, Hammock Grove is free. On weekends it can get a bit busy, so the hammocks are first come basis.
I think if you are on an early ferry it's free. There is no charge for the trip to Manhattan.
The ferry charges $2.00. That's all you pay. If you have a NYC ID it's free.
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