It's much more expensive to go by sea. There are no 'ferry' services, per se.
You either have to pay to travel on a luxury cruise liner
http://www.cunard.co....nsatlantic-Crossings/
or you have to travel on one of the few freighter which carry passengers. They can take ages (often going via other European ports, such as Amsterdam or Lisbon, before they cross the Atlantic), with fares running into thousands of dollars:
http://www.freightervoyages.eu/Routes.htm
Chris