Go here http://www.mass.gov/eea/agencies/dfg/fba/access-sites.html
se the search bar under the access points. Type Merrimack. I've only used the Lawrence and Haverhill ramps. Lawrence has more parking. Both ramps are busy on the weekend. Good luck
Concord River between Bedford and Billerica. Launch in at Rt. 225 in Bedford. Every now and then you hear pike caught in the Merrimack which usually come from the Concord River.
+1 for the Concord River, I've only fished from shore but I have caught numerous Northern Pike there. In fact I fish exclusively with steel leaders while at the Concord even though I am targeting bass. I've caught pike from both the Rt 225 launch in Bedford/Carlisle line and in Billerica near rt 3a.
My 1st pike ever was at the Concord on a 10" berk power worm, and then my 2nd pike the same day on the same lure. Since then I have caught them on spinnerbaits and series 4 SK crankbaits. My brother also landed a nice one on a bass jig/craw trailer.