I fill my boat with friends and anchor on Lake Union every July 4th. I think it's a blast, but it's not for everybody. It can be crowded, chaotic, and full of drunk idiots who haven't a clue how to anchor, but for some sick reason I love placing myself right in the middle of the chaos. We bring out kayaks, the dinghy, and other water toys, spend the afternoon swimming, paddling around the house boats, and BBQing, then we've got a front row seat to the fireworks barge at night. Then after the show, you can head back out through the canal or stay on the hook for the night. July 4 is the one night per year when overnight anchoring is allowed on Lake Union.
Now if you're looking for dock space with a good view of the fireworks, that's a lot tougher. You might call the Center for Wooden Boats on south lake union and see if they have anything to offer or any advice.