Underwater Dock Inspection: The Annual Check Most Owners Skip
By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.
Dock & Pier Repair | Underwater Inspection Last Tuesday I was down at a private dock in Charleston Harbor, and the owner told me he'd never once had the underwater part checked. He'd bought the house in 2003. I put on a mask and rolled over the side. Ten feet from the steps, a 12-inch pile had been chewed down to about half its diameter. Marine growth was hiding it completely. He was standing right there. I just pointed at the video screen. Most people see a dock as a deck that happens to be over water. They inspect the boards, they paint the rails, they replace the stringers. But the piles that hold it up are doing the hardest work, and they're the hardest to see. That's why the annual underwater dock inspection is the check most owners skip. This isn't a luxury. It's the difference between a quick fix and a full rebuild.
What I look for on every underwater dock inspection
When I dive a residential dock, I use a Kirby Morgan helmet and a video camera. I know some guys work with just a mask and a slate, but I need images to show the owner. A picture of a rotted pile beats a thousand words. On timber piles, I'm checking for marine borers, rot, and mechanical damage. I run my gloved hand over the whole surface. In clear water you can see the shotgun holes from teredo worms. In murky water, you have to feel for soft spots. I use a sounding hammer, tap along the pile, and listen for the dull thud of hollow wood. On concrete piles, I'm looking for spalls and exposed rebar. On steel, I check the coating and measure web thickness with a caliper if I can reach it. Under the bottom, I check for scour. A pile loses its strength when the sand is washed away. I measure the exposed pile height below the mudline and look for any hardware: through-bolts, ties, pile wraps. You'd be surprised how many brackets get missed because they're underwater.
Marine borers, scour, and the one that scares me
The most common problem I find is marine borer damage. Teredo worms, though they're actually a clam, drill into the wood and turn a solid pile into a honeycomb in a few seasons. Call it two to ten years, depending on the water temperature and salinity. I've seen it from Florida to Louisiana in warm brackish water. The wood looks fine on the outside, but inside it's gone. Reminds me of a job in the Lowcountry where a guy had a dock built with treated southern yellow pine. Not marine grade. The borers had done so much damage you could poke a broomstick through a pile. He kept saying 'I thought pressure-treated was pressure-treated.' There's a difference between ground contact and marine rated. Scour is next. Propwash, current, storms. A pile driven twelve feet into the bottom might be sticking out eight after one big storm, give or take. I've seen piles held in place by nothing but mud at the toe. And the one that really scares me is a broken pile being held together by marine growth. A thick coat of barnacles and oysters can hide a crack completely. You look at the video and it seems solid. Then you brush the growth off and the thing is snapped. That's why I always scrape a section of growth away on a few piles, especially near the waterline. It's an ugly job, but it's the only way.
Do it yourself or hire a diver?
If you're wondering about doing a dock inspection underwater yourself, here's the hard truth. You can buy a GoPro or a pole mount and see something if the water is clear. But clear water is the exception, not the rule, and the stuff that'll kill a pile often hides behind growth. You can't feel soft wood through a camera. A screwdriver and a hammer are better, but only if you know what you're looking for. My recommendation: hire a commercial diver once a year. A guy who does this for a living will have a video system and an inspection report. He'll mark the piles, photograph them, and tell you which ones are acting up. The exact cost depends on the site, but it's a fraction of what an emergency repair will run you. If you do want to do a basic surface inspection, go ahead. Walk the deck, look for any place where wood meets water, check for cracked fasteners. But understand that what's below the waterline is a different world. You wouldn't skip your roof just because the ceiling looks fine.
What to do with the report
If the dive finds active borers, don't wait. A pile wrap can stop them if you catch it early. If they've honeycombed the pile, you replace it. If scour is the problem, add riprap or a concrete mudmat, but get a professional to confirm the fix. If it's just some cosmetic checking, mark the pile and re-check it in six months. Keep a log. I keep a waterproof notebook per job. Date, tide, water temp, findings. That baseline is gold in five years. The point of the annual check is to catch things before they become emergencies. A dock is a structure, just like your house, but you can't see half of it. You can call the right people, ask the right questions, and sleep easier. The one thing you don't want to do is nothing.
Frequently Asked Questions
How often should I get an underwater dock inspection? At least once a year. If you live on a channel with heavy boat traffic, or the area gets tropical storms, go back and have a look after any big weather. A post-storm check is cheap compared to a new pile. Can I do the inspection myself? You can look from the surface with a pole or a camera, but the important stuff hides below the waterline and behind marine growth. Hire a commercial diver. If you insist, at least get in the water with a knife and scrape a few piles. But a trained eye will catch things you miss. What does an underwater dock inspection cost? The pricing model is simple: hourly rate plus mobilization, or a flat fee per pile. The exact number depends on your pile count, water clarity, and current. Get a few local quotes and you'll see it's not a big line item. Compared to a collapsed dock, it's pretty close to free. What's the biggest danger to dock piles? In warm water, marine borers. In strong current or propwash, scour. And in older docks, a mix of both. An annual dive will catch all three before they become structural problems. If you wait until you see trouble from the deck, you've already lost.