How about building the dock platform first using some barrels as floats and then pushing it into the water. Positioning it where wanted. Then depending on your desired post you could put some guides on the side of the platform. then put your support posts set in concrete with rebar sticking about a foot out of the bottom of a 5 gal. bucket.
Then you just drop your post using the guides and a sledge hammer and Presto you set it into the hopefully soft pond bottom!?
