As Grant mentioned, I took the Cunard ferry (Queen Elizabeth II) from New York to Southampton. It was an interesting experience - I have always wanted to cross the Atlantic by liner - but a lousy way to ship a motorcycle.
Air freight is simpler, faster, and - cheaper. I recommend you ship to either Toronto or Montreal in Canada, rather than directly to the United States. It costs less to ship to Canada (about 40% less), and you don't have to deal with all the paranoia and fear, uncertainty and doubt associated with air travel to the USA. I have used Motorcycle Express in the past, and have always been very pleased with their service.