If you're interested in taking a cruise ship to America, as well as to other interesting ports of call, a good specialized travel agency is Cruise Web (www.cruiseweb.com).
You can also travel to and from the USA on a freighter, though it will be much slower and less cushy than a cruise. Nevertheless, freighters aren't spartan (some advertise cruise-ship-level amenities), and they are much cheaper (sometimes by half). Trips range from a week to two months; stops at interim ports are usually quick.
For more information, try Cruise and Freighter Travel Association (www.travltips.com), which has listings for freighter cruises and other boat travel.