I know the original poster asked for places on property, but another place to get good lobster and New England style seafood is the Celebration Town Tavern.
Obviously Maine lobster is going to cost a bit more than you might pay in Maine, but at the Tavern, you aren't paying the premium for being on Disney property.
And it is a NICE place too. In warm weather, you can sit outside at one of the covered gliding "picnic table" seats.
Ted