well, there's not that much in the way of excellent seafood here, as we are about as landbound as you can get, but my wife swears that the lobster roll available at williamsburg snack bar is one of the best she's ever eaten anywhere, and believe me she has eaten her share. it's only available fri - sun, but the roadside shack also has nice dogs + burgers + whatnot. i am parricularly keen on the williamsburger, which is a patty topped with coleslaw + a couple of onion rings. the place is located at 93 Main St Haydenville, MA 01039, which is route 9, west of northampton. a good spot.
my fave diner in the area -- for food, not necessarily the decor -- is either the look restaurant in leeds (410 N Main St
Leeds, MA ) it's open breakfast/lunch 'til about 3 PM i think and has solid diner fare. this is also on rte 9, west of northampton (these small towns run into each other). there are a few more classic diners around -- bluebonnet in northampton ( (324 King Street Northampton, MA 01060-2333 (413) 584-3333; 6 AM - 11 PM daily) which i'm not crazy about food-wise, but it's a hep place. there's also the miss florence diner (99 Main Street Florence, MA 01062-1478 - (413) 584-3137) -- this is also rte 9 west of noho -- which also has a better look than food. both these spots might be best for a cup of joe and a look-see. there us also kathy's diner in northampton (6 Strong Ave Northampton, MA 01060) which i haven't been to since it was the red lion. but it's there and i think is still open for breakfast/lunch. the only other proper diner right local is the whately fillin' station ( (372 State Road
South Deerfield, MA 01373 (413) 665-3696) which is open 24 hours. again haven't been for a while, but it's okay. there are a good selection of burger/dog places locally, but you didn't ask. the old school seafood places that were stand-by's around here are long gone. closest current approx is webster's fish hook in northampton (391 Damon Rd Northampton, MA 01060 413-586-3190) but i haven't been there since they listed their french fries as freedom fries.
for amherst, there's not much excellent roadfood fare in the downtown area. but there's a very good irish pub/fish & chips place just to the north of downtown -- the harp ( it's especially nice in warm weather, since they have a deck out back. and if you like beer, i highly suggest the moan & dove in south amherst ( it has an outpost in northampton as wellm called dirty truth ( dirty truth has food as well -- quite good pub fare -- and can be tad hard to find as there's no sign out front.
hope this helps