Enjoy the Austin Symphony''s rendition of the 1812 Overture ( including canon) and get an unobstructed view of the fireworks from the lake. 
You have to either have your own boat, or make your own rental arrangements for this event. I believe there is a rental livery in the Holiday Inn but I am not sure if they are open or what they charge. We used to put in at the TRC dock, but since they now close off parking at Austin HS for the shuttle buses it''s no long convenient. Parking will be easiest at I-35 boat ramp and it makes the paddle down stream on the way back.
Its a good, brisk 30 minutes to an hour (depending on current) paddle on the way up, but it should be an easy float on the way back (expect it to take an hour though because of the crowds).
Contrary to what is reported in the paper the lake is not closed, only the section immediately under the fireworks is closed. However the police patrol the lake to be sure to have a life jacket in the boat, and some kind of illumination - bring a light for your boat.
The good parking spots go fast so you may want to get there early and picnic at the boat launch. Last year all the good spots were gone by 6:00. I plan on getting there around 4:00PM and having a lakeside picnic.