I have built several docks using 6x6's placed on the bottom of the lake and it has always worked fine. I always screw a 5 gallon bucket to the bottom of the 6x6 and add pieces of bricks or rocks to the bucket so that the bottom end of the 6x6 won't float. The bucket also gives you something to hold onto if you need to move the 6x6 around in the lake. I have also used 4x6's and they worked as well and are obviously cheaper. Make sure that you provided cross supports for the 6x6's to stabilize them and use some additional support for any long spans in your dock.
