You can access Buttermilk Bay from the Marina adjacent to the Merchant Marine Academy orfrom the launch at Makos' Bait shop. Both charge, the marina charges $10/launch and I don't know what Mako will charge.
Don't overlook the stretch of water just infront of MAKOS and between the two bridges. There are very good size fish holding around the Rte 28 bridge on an outgoing tide or at slack. and it gives a thrill to all the bait guys at Makos' to see someone pull a good size one in on a fly. I write this from Seoul Korea and can't wait to get back home to Plymouth in another week to catch up/
good luck