Wade fishing is excellent on the Merrimack River. Joppa Flats is about 1 mile inside the mouth and can be accessed from a simple boat ramp with free parking off of Water Street in Newburyport. It is wadable only 2-3 hours on each side of low tide. Another spot is the mouth of the river at the north end of Plum Island, where you will find heavy currents running along a sand beach and over an ever-changing sand bar. Just cross the bridge to the island and go to the north end.
Parking is plentiful.
Surf fishing opportunities along the front of Plum Island are plentiful. You can also proceed to the southern end of Plum Island, known as Sandy Point, and explore Emerson's Rocks along the beach, the sand bars at the tip or the flats and mussel beds on the backside. Some parking restrictions may be in effect at certain times on the refuge and at Sandy Point, so check at the gate when you enter. The best boat ramp for accessing the Merrimack River is Cashman Park in Newburyport. Recently upgraded, this state of the art ramp puts you onto the river just above the Route 1 bridge from which point you can go upriver or down.