The BLT bridge has the benefit of being steps away from other areas you can watch fireworks that have reasonably good audio - such as the CR 4th floor observation deck and (my recommendation) head up a flight or two on the exterior stairwells of the building and watch from there. I like that this area provides options to move around/position yourself based on what you encounter in terms of crowds on any given evening.
Not sure what the latest is with GF boat dock show audio - hopefully someone can confirm.