Every dock I have built at my place has been using 6x6's and/or 4x6's with a 5 gallon bucket nailed to the bottom and filled with rocks or bricks. As long as the bottom of the lake is not mush, they won't settle much at all. The bucket with rocks will help to hold that end down on the bottom and the bucket will give you something to grab onto when you need to move the posts around to level the framing up. I use cheap treated landscape timbers to add additional stability to the dock as I get it framed out. I normally nail or screw a piece of scrap wood to the bottom of the timbers to provide a base for them. I do use a sledge hammer to force the timbers into the bottom. My land is mostly sandy red clay so that it is a pretty firm surface on the bottom. You can only work this way in water about armpit deep.