I've seen people get plastic drums, cut them in half, fill them with about 12" of concrete. The concrete is left to dry. The half barrels can be rolled to the lake shore and floated out to where they need to be sunk(area pre flattened). The pressure treated poosted are then set on top to build the deck. The concrete provides a good base, the plastic barrel keeps the concrete together and makes it easy to move around, the diameter of the barrel provides a big footing.