The Cashman Park ramp in Newburyport is great.
http://www.mass.gov/dfwele/pab/pabpdfs/pab-75.pdf
Get there early on a Saturday or Sunday, however.
I would go take a look at the river at low tide to familiarize yourself with the rocks and bars in the area and then come back and fish it on a dropping tide.
Lots of guys have luck on the incoming but I have generally had better luck on a dropping tide.