We normally ride the Harley down to watch the Alton Bay display on the 3rd rather than go by boat. So my perspective on watching from a boat is only as an on shore observer.
Docking at the Alton Bay public boat slips fills-up very early.
As fireworks are launched from a barge usually slightly north of the floating bandstand boat anchoring can be either lower in the bay or up bay from the barge and outside of a safety perimeter.
Plan on being at anchor for quite a while in terms of personal comfort.
If it is a nice evening there will be a
lot of boats. We normally hop on the bike right after end of show and riding north we can view the mass exodus of north bound boats. As has been noted you have to be ready to deal with that in terms of navigation.
Have Fun!