A seacock is the valve on a hole in the hull below or near the waterline: engine cooling intake, head, sink drain, cockpit drain. Open, it lets water in or out. Closed, it's the only thing that stops the sea if a hose or strainer downstream fails.
A proper seacock has a flanged base bolted through the hull. ABYC H-27 asks for seacocks that are readily accessible, corrosion resistant and strong enough to hold a 500 lb static load for 30 seconds, as Nigel Calder quotes in Good Old Boat.
What a surveyor checks
Every handle worked through its full travel. The body and the hull fitting from inside, and the outside face on the haul-out. Pink, dezincified brass, which looks sound and crumbles. Double clamps on the hose. A ball valve threaded straight onto a through-hull gets noted as a weaker assembly.
In the report
"Engine room, raw water intake seacock seized in the open position. Free or replace before the vessel is next underway." A seacock that won't close is usually a safety finding. See seized seacocks.