1. It's this pier in Hoboken (near Sinatra Dr and Constitution Ct.) that appears to be part of some marina. It doesn't look like a public park, although I'm not very familiar with Hoboken.

2. You can get there via public transit, although this particular location isn't nearly as easy as many other good skyline viewing places (not to mention the issue of getting access to a private pier). Judging by this unofficial NJ Transit map, NJTransit buses 22, 23, 89 and 126 all pass within a block of two. You can get schedules here, although they don't list all the stops, so it's hard to know where exactly the stops are. The Hudson-Bergen Light Rail also stops not terribly far, although it would be a bit of a hike.

3. You might consider taking a ferry from the W 39th St pier to Paulus Hook in Jersey City. That will give you a great view of the midtown skyline from a variety of angles on the ferry, and then from the Paulus Hook/Exchange Place area in Jersey City, you'll have great views of the downtown skyline, and can easily hop on the PATH train back to Manhattan.

4. There's a new W Hoboken opening soon. There's also the Sheraton Weehawken, which should have similar views. Probably not as good views of midtown, but still excellent views of downtown from the soon-to-open Westin Jersey City as well. There are also Doubletree and Hyatt hotels in Jersey City with excellent views of downtown (the Hyatt is right at Exchange Place).