A through-hull, or skin fitting in Britain, is a fitting that makes a hole in the hull: the engine's raw water intake, the head's intake and discharge, sink and cockpit drains, speed and depth transducers. Below the waterline, most carry a seacock so the hole can be closed.
They're usually bronze or a glass-reinforced composite. Metal ones sit in the water with the boat's other underwater metal, so they're part of its corrosion story. Whether to bond them to the boat's electrical system is one of the old arguments in boat wiring: Don Casey and Nigel Calder take different sides.
What a surveyor checks
Each fitting from outside on the haul-out, looking for pink, dezincified metal and cracks. The seacock and hose from inside. Bonding wires, if fitted, for broken or corroded connections. A fitting that's pink from stray current is a safety finding.
In the report
List each through-hull by location and purpose, then the faults. "Galley sink drain through-hull, bronze, pink and pitted on the outer face. Replace with the seacock at this haul-out." See through-hull bonding.