We took a few friends out and watched the firework from the water
a year ago and had a blast! I've not gone out to watch fireworks on a 4th but there were less boats than I expected. Around us were a number of larger party boats, and scattered small boats. We were kept at about 100-150 yards from the firework platform so it was fairly close to the action. It doesn't really matter where you are the fireworks seems to be going off right above you. Like other on-the-water events
most people weren't anchored in, myself included. It wasn't extremely crowded and the show really doesn't last for more than, maybe 10 minutes? If you prefer you can keep some distance from the crowd and find a wide open area so you wouldn't have to constantly be focused on maneuvering as I was. It was the first time I've seen the NYE firework that close after having lived in SF for the last 15 yrs and I definitely recommend it. Plus the night time cityscape was quite a view!