Believe it or not, some cruise liners actually stop in Boston. You can catch boats to Bermuda, Canada, Miami, Maine...and many other places apparently. Cruises here dock at the Black Falcon Terminal in the soon-to-be-glamourous Seaport District. Since the area is somewhat industrial and blighted, your cruise will probably get some kind of bus to whisk you off to the city centre.
Cruise ship passengers arriving at South Boston's Black Falcon terminal get a free bus service to the city centre.