There are two ways. First is taxi. There will be plenty waiting when your ship arrives. It's about a 10-minute ride to the airport barring any traffic jams. There is also mass transit to the airport, but you'll have to do a little bit of walking and make a transfer. The MBTA Silver Line
will get you there. It's about a 2-block walk to the SL3 line, and then transfer at South Station for the SL1 line.
Best bet, IMO, is to take a taxi.