There is a multi-story parking garage that was managed by Kinney Systems (mayed have changed managemen)t. It is just a short walk from the embarkation area. I believe the weekly rate was comparable to airports, at about $75 for a week. Do Print out the map to get there as it is right in the middle of the Big Dig area. I was just in Boston mid-wek and it is still a mess and very time consuming to get around the streets in that general area. Be patient and allow ample time to get to the port. Once you pass South Station, you are only about a mile away and through the worst traffic area.