Did a year fellowship in Boston. Great city to visit. Spring weather is usually pretty crappy though... cold and dreary, so dress warm and bring an umbrella.
As for seafood, Legal is a chain, and is good, but I would recommend you try The Daily Catch. Faneuil Hall has a few places with really good clam chowder as well.
You absolutely have to go to the North End. You can pretty much pick any restaurant on Hanover St and get great Italian food. My favorite is Giacomo's (pronounced Jock-uh-moes if you ask anyone where to find it). It doesn't open until 5, ONLY TAKES CASH, and only has like 12-15 tables. People will wait in line for 45+ minutes for a table, and it's worth it. One that's a little off of the more crowded areas is Massimino's, and is also excellent. It's closer to the Garden. Hit Mike's Pastries or Bova's (a nurse who grew up in the North End said Bova's is the best bakery in town) for dessert.
Pizaria Regina is the best in town, also in the North End.
Other very good restaurants:
The Barking Crab. Popular place, lots of people to watch, and good.
No Name Restaurant. On the fish pier, fried seafood, VERY relaxed feel.
Grill 23. High-end steaks. Picked by Men's Health or something as one of 5 best steak restaurants in US while I was there.