If a boat dock is not maintained correctly, the posts securing the dock to the floor of the body of water will erode and deteriorate, necessitating potentially extensive repairs. You may need to contact a professional depending on the severity of the damage and the potential danger of the repairs required. If the water is safe, repairing a post on a boat dock is straightforward. You'll need to get pressure-treated lumber for any posts you replace to resist damage from the water.
Inspect the portions of the dock posts that are above water level for minor problems such as loose nails. Put on your goggles, snorkel and flotation device and place your face in the water. Use your hands and vision to identify large problems such as loose, rotting wood, excessive marine life growth and displaced posts.
Treat any wood that will replace posts in the water with deck sealant. Apply at least two coats according to the manufacturer's instructions and allow the sealant to dry. Place the new post beside the existing one in the water. Turn on your water pump and use its hose to displace the sand at the base of the replacement post, allowing it to sink into the sand bed. Drive the post at least 4 feet into the bed floor. Use your water pump to move sand and sediment around your new post to hold it in place. Secure the new post to the dock with galvanized nails using a hammer or a nail gun.
Pry nails from the old post you're removing from the dock. Use the water pump to displace sand from the base of the old post. Rock the post in a back-and-forth motion while using the water pump to loosen the hold of the sand bed, working until the post dislodges. Bring the post to the surface of the water and dispose of it.
Perform minor repairs to dock posts such as nail replacements using galvanized nails. Apply sealant to any dock posts you did not replace, following the manufacturer's instructions. Do above- and below-water visual inspections annually.
Always use a flotation device approved by the U.S. Coast Guard, and never swim alone.
Do not remove an old post from the water before installing a new post. This may cause the dock to collapse because a weight distributor is being removed and will place unnecessary strain on remaining load-bearing posts.