Just another idea. A few years ago we used 3" square tubing 1/4" thick for our post. We made a driver that would slip over the post that we could drive it into the lake. The driver was made with a handle on each side that we could pick up and let it fall. Kind of like what they use to drive tee post in the ground. After squaring up the sides it made it easer to drill the holes to mount the boards on the sides. A lot easier to clamp to when something is flat rather than round. We have made two docks this way for different members and works well for us. Just another thought.