DC is the last place to go by sail to see the fireworks. You really cant get close enough and the marinas will all be obscured by the federal buildings. If you want to watch them on the land...say the National Mall Area between the US Capitol and the Washington Monument be prepared to join 300,000 of your closest new found friends in a trapped security enforced zone. This is the best place to see them by the way. They are the best in the area, but you pay the price trying to leave afterwards. I saw them 2 years ago from my officr, which is in the US Capitol and there is no other place like them, but the hassle and throngs make them unenjoyable. Its kind of like see the ball drop at Times Square...you may only do it once,
T37Chef is correct, the best place to view them is on the Bay and many of them are almost the equal of DC. Ft McHenry/ Baltimore is a great place to see them from your boat. Rock Hall and Annapolis is also is also and they usually do them all on different nights. One year we went from anchorage each day to anchorage and saw 3 different firewaorks on 3 different nights.
Hey Chef...have you ever seen what the community across from me does on July 4th night. They have quite a show in each of the neioghborhoods...is great to anchor in the cove off of our docks and watch it.