Take Route 134 North until the end where it intersects with Route 6A. At that light go straight across onto Bridge Street. Follow the blue signs from onto Sesuit Neck Road The "West Side" of the marina is on your right. This lot is primarily used for Resident sticker holders and daily parking.
There is also an "East Side". Simply follow Route 6A towards Brewster. Turn left onto South Street (directly across from Airline Road), follow the blue signs to get to the parking lot on your left. This lot is primarily used for Non-Resident sticker holders and daily parking.