How growth builds
First, a thin film of slime. Then algae, thickest near the waterline where the light is — the green or brown beard owners notice first. After two or three months, barnacles and oysters settle, especially where the bottom paint is weak, and they hold on hard.
What you can do yourself
The waterline you can wipe from the dock or a dinghy with a soft brush or a long-handled hull brush. Keep it soft: hard scrapers and metal brushes take the bottom paint off along with the growth.
That keeps the boat looking clean, but most of the growth is below, out of reach — on the flat of the hull, the keel and the running gear.
Why the bottom needs a diver
Propellers, shafts, struts, rudders, thrusters and water intakes are all under the boat. Barnacles on a propeller cost speed and fuel; in a water intake they can make the engine overheat. Getting them off takes time underwater, the right tools for your paint and air to breathe.
The risks of swimming at a marina
Faulty shore power on a dock or a boat can leak electric current into the water — one reason many marinas don’t allow swimming. Add boat traffic, poor visibility under the hull and barnacles sharp enough to cut. If you do go in, never swim near boats plugged into shore power.
When to call a diver
When the growth is past slime, when you see barnacles, before a trip — or simply once a month. With regular cleaning it’s the standard price. About two months since the last cleaning, × 1.5; with barnacles and oysters after three months or more, $150 per hour with an estimate first.