My dad and I just finished building the frame for our dock and we had the same problem trying to figure out how to get the posts in with the lake full. First thing thing we did was plant the supports on shore with concrete. Then to attach the 2x8 boards for the sides we took our flat-bottomed boat and tied the front to the shore and pouded in 6 1" galvanized conduit into 2 tripods (wich were very solid) 1 on either side. Then we put a loop of rope around each of those to hold the 2x8's in place while we bolted them to the supports on shore. Once we had done that all we had to do was go out into the boat and pound in the 4x4 supports about 3'-4' into the clay and bolt the 2x8 sides onto them. It wasn't really all that hard, just a whole lot of checking and re-adjusting with the level. The trick was using the conduit tripods to hold every steady and in place so you could pound the posts in.
-Scott

Take great care of it, or let someone else have it.