J.S.
We lived in DC in 2004/2005. Because we were IN the district, there we decided that it was a life experience not to pass up. That said - we did not want to deal with the crowds (we had a newborn) so we opted to drive down really early on Saturday (I think we left our apartment at 6:00 a.m., so after parking we were there before 7:00) morning and park near the mall - then we walked over and did our own walking tour and took photos of the trees around the tidal basin.
I am so glad we did it - it was amazing! That early in the morning it was not busy at all... like not even kind of busy. The weather was cool and very comfortable (I wore a fleece vest and jeans) and very nice for walking. We walked all around the tidal basin, took tons of photos of me and the new baby. I love the images and it makes a very special keepsake for my son (the only of my children born in D.C.)
Where does the cruise take you? I think it might be hard to see them from a boat. Walking under the trees, breathing in the scent, taking photos close up was the experience for me. I don't even know if you could really see them from the river... unless the boat takes you on the tidal basin, but that seems weird to me.
We did not do any of the other festivities for the Festival - my son was only 2 weeks old and we did not want to deal with the chaos :)
I'd vote, if you go - go early and walk.
Also - Dumbarton Oaks in Georgetown is AMAZING in the spring. If you are going into D.C., it's worth trying to do both.
Good luck!
J.