Chowing in Newburyport
It sounds like the Black Cow is the place for you. The food is consistent, you can go casual, and it sits on the water. It's a very popular restaurant. (The original in Hamilton was such a hot spot, they built this second one). The menu is varied - everything from great burgers to fresh seafood and pasta dishes. The link below has directions:
Scroll back through notes on Boston Area to about June 1 for "Best North Shore Seafood" or something like that. References to Newburyport, Salem, Essex. Newburyport has lots of restaurants. If you want to do a "t" and shorts at a clam shack, hustle out of N'port to the Clam Box in Ipswich. Be warned: unless you go in an off, off hour, prepare to stand in line.