Rather than drive and park (both difficult),
you may wish to take the
Boston Express
bus from Nashua to South Station,
then walk to Long Wharf (from where the
Boston Harbor Islands
ferry departs
to Spectacle Island and George's Island).
Google map: Nature Walks in Northeastern MA
Boat travel may be limited in winter
There are fees for the Nashua-to-Boston
bus and the Boston Harbor Islands ferry
Entrance onto Spectacle Island itself is free