Jamesport Ramp
To use the Jamesport Ramp you need to pick up a Riverhead town parking permit, at Warrens Tackle or Riverhead Town Hall. It is pretty inexpensive though, $5-10 for the season.
I was out on the North Fork today and checked the Mattituck Ramp. There are two ramps there. One for Mattituck residents only and one for Southold town residents. The non-resident permit is $50 for the year, and it gives you access to a lot of the Sound and Peconic Bay.
There is also a state ramp for very small boats (16 feet and under) at Oyster Pond. East of Greenport on the neck before Orient.
Chris