Bruce,
I kicked around exactly how to do this at my place. I gave up on the treated posts and decided to set galvanized. My dock is 4' x 24' so not the width you have. I put a ladder in the water and sledged the heck out of them. I bet they went 2' into clay/much if not more. I belled out the tops of the posts pretty bad, but easily clean cut them off with a pipe cutter when the project was done. I bet you can space posts inside of your 10x10 frame and have them not protrude the decking. I'm very happy with the stability of the dock.

I would have rather had treated posts so hopefully someone has some ideas. Another dock I build years ago was done when I completely drained the pond so we had some really nice footers, forming tubes, and the 6x6 posts. That baby was solid.