Definition
A seacock is a valve fitted at a through-hull fitting that opens or closes the connection between the sea and a vessel's systems, and the engine's raw-water seacock is the valve that admits cooling water.
The raw-water seacock is the single most important valve on the engine, for two opposite reasons: an engine started with it closed destroys its impeller within a minute, and a hose failure downstream of an open one floods the boat.
Everyone aboard should know where it is
Not just the owner. In a hose or strainer failure, the time it takes to find the right valve is the difference between a mess and an emergency. The same applies to the fuel shut-off.
Operate it regularly
Seacocks seize when left untouched. A valve that has not moved in three seasons may not close when you need it to, which is exactly when you need it. Operating them through their full travel periodically is basic maintenance, and inspecting the through-hull and its fastenings belongs on every haul-out.
Where you meet it
In pre-start checks, in lay-up procedure, and in the instruction to close it before extended cranking.