First, by removing the algae slime layer before invertebrate (Hard growth) can attach and dig into the bottom paint.
Second, cleaning the painted surfaces is only part of the job. The diver must also tend to clean the unpainted metal parts of your vessel, primarily the running gear and thru hulls. Proper cleaning of the hull, running gear and thru hulls improves vessel performance and saves fuel costs. Keeping the thru hull intakes clear ensures proper performance of the various systems inside the vessel, engine, generator, head, air conditioner, refrigerator and galley.
Third, another extremely important service provided by the underwater hull cleaning diver is tending to and replacing the sacrificial anodes (Zinc). The diver must inspect, tighten and periodically replace the various zinc anodes which protect the different metal parts of the vessel from galvanic corrosion (electrolysis).
Fourth, during the hull cleaning process the diver inspects the hull and hardware as he cleans it for impact damages and electrolysis. He also monitors the fiberglass hull for osmotic blistering, the wood hull for worm damages and the steel or aluminum hull for electrolysis.