Can't really help you with any of the details as I have never been there, but I would tend to agree with the person who said it sounds like a hassle in these post 9/11 days. The kayak idea might be good...then you'd be away from the crowds. I've done the 4th in Boston a couple of times (a friend wanted to go and talked me into it) and because I was familiar with the area and ready to move fast at the end of the fireworks, was able to strategize how to avoid the worst of the mob scene. However, I don't think I would go to an event like that in an area I was not familiar with--too stressful to be stuck in a crowd and not know where to go. If you'll be with anyone who knows the area they might be able to help you avoid that scenario. Otherwise, I second the recommendation to watch it on TV!
2011 Surly LHT
1995 Trek 830