If you are just coming in the night before the cruise and will not have time to explore Manhattan anyway, you might want to consider one of the much less expensive hotels right at the Newark airport area, which free airport to hotel shuttle. The same shuttle service as I mentioned above will shuttle you from Newark airport to the NY cruiseport for the same price as above.For example, the Sheraton Newark airport hotel starts at $155 per night for Aug. 7.
(I just noticed you will there two nights). You can also, if you choose to stay both nights in this area, take the Air train to and from the city on the 8th, , which only takes about 20 minutes. There is also a less expensive but more time consuming Newark Airport Express Bus that takes you from the airport to four different stops in Manhattan. Round trip on that is $22.