Here is a list of most public launch ramps in the state:
http://www.mass.gov/dfwele/pab/pab_facilities.htm
Cashman Park on Merrimack Street is a city-owned, all tide facility.
http://www.mass.gov/dfwele/pab/pabpdfs/pab-75.pdf
During the June - August season it is very busy on the weekends and will fill up. Get there
early.