Seafood in Belfast
I don't get out to the Belfast area much. What I do know is that in Maine along the coast, you'll see a random truck or van on the side of the road selling seafood. Some of them may look a little shady, but most have pretty darn fresh seafood.
I live in Rockland and work in Camden, and I go by three different "trucks" on Route 1 on the way to work. I've got nothing but good stuff from Delano's (near the Samoset intersection) or Dad's Seafood (right in between Rockland and Rockport).
If you're looking for more of a storefront place, you may want to check out Jess's Market in Rockland. Occasionally we'll go through Jess's at work for special order stuff (monkfish, etc), and its all been top notch quality.
Regarding dining suggestions, again, I'm not that familiar with Belfast. If you're willing to drive a bit, here's a few places to look up:
Hartstone Inn, Camden (where I work :p )
Francine's Bistro, Camden
The Edge, Lincolnville
Hope this was helpful, enjoy your trip!