Boat or Dock?
Boat wise, I am not to sure for the GSB area, but dock wise, there is the West End dock in Sayville off of Main St. or if you want to head down to Bellport, I know a very nice spot where I just saw them knocking the crabs dead a week ago, Bellport Bay. If you PM me I can find out the name of the street it is off of again as it slips my mind at the moment.
If you have a boat, should be easy at night to just cruise the back bays with a light and a net.
Nothing better than a good crab sauce....