Chowing in Newburyport
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.
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: