A seacock is a valve on the hull of a boat or a ship, permitting water to flow into the boat, such as for cooling an engine or for a salt water faucet; or out of the boat, such as for a sink drain or a toilet.
Seacocks are left open or are closed depending on the situation. Seacocks feeding into or out of a closed system, like the engine cooling system, are almost always left open. Seacocks connected to something open, such as a sink drain, might be opened up in port but closed when at sea. The reason for this is that when the boat is level, the drain or other opening will always be above the waterline, and so water will only flow out. At sea, when the boat rolls in the waves, the opening may sometimes be below the waterline. If the seacock is open, water may flood the boat.
- Seacock definition in InterSailing Glossary