Ferry leaves from EDIC dock in the Marine Industrial Park in south Boston, near the Black Falcon cruise terminal.  It is a little tricky to find, so leave extra time. There will be an orienteering sign at the dock.  Directions to the dock can be found at http://www.thompsonisland.org/about-us/directions.  The dock is accessible by T, but if you drive, it's advisable to carpool as parking is in a parking garage (~$10?).  Ferry service from EDIC dock will be free for orienteers; it takes about 30 minutes to get to the island.  Ferry schedule is:

Leaving EDIC dock:

11am, special boat for orienteers, please take this one if possible

12pm (if anyone asks, mention you are there for orienteering)

Leaving Thompson Island:

2pm

4pm