It's literally less than a 10 minute walk from the Boston Cruise Port to the waterfront, with lots of attractions like restaurants (Yankee Lobster) or Harpoon Brewery, and 5 minutes more to tons of shops, siteseeing boats, etc.
And the Silver Line of the MBTA is RIGHT AT the cruisedock. Less than $3/ride, gets you to Faneuil Hall or the Freedom Trail or Fenway Park, etc.