It's absolulely doable. A taxi will take 10 minutes. Public transportation will take 30 minutes or less.
Assuming that you're taking the Bay State Cruise Company ferry (I'm not even sure if there are others), here's their recommendations:
http://www.baystatecruisecompany.com/directions.html
If you arrive on time and are looking to save a little money, the Silver Line bus definitely is the way to go. If you take a taxi, make sure the driver takes the Ted Williams Tunnel to the South Boston exit; Boston cabbies love to take roundabout routes!