Make sure you have plenty of small bills on hand for tips. Recipients would include bellhops and other baggage handlers (about $2 per bag in NYC), and the hotel maids (about $4-$5 per night). If you plan on taking taxis in NYC, note that not all of them accept credit cards, so be prepared.
How will you be setting up your shipboard account...do you plan to settle in cash, or have it put onto a credit card. This would also impact the amount of cash to have available.