Which pier will you dock at? If it's Black Falcon you are about a 45 minute walk along the historic waterfront
There is a self guided tour,,,,Called "The Freedom trail"
With maps and instructions.....http://www.nps.gov/bost/freedom_trail.htm
If you have the time try a "Duck tour" which is a tour of the city and then a trip on the water .
If you must cover ground try a taxi,,,,,,if you have good shoes and many hours try the walk,,,,but as in any larger city,,,be carefull and dont get lost,,,,,