Sugar Pine Boat Plugs, 4" Long x 2" x 1"

If you have thru-hulls you should have a set of these wood plugs for use in an emergency. They can be used to stem the flow of water in case of hose, valve, seacock, or thru-hull failures.
Should a plug be too long for a particular application, its length can be modified with a saw.
When used to plug the orifice of a damaged valve or thru-hull, after a period of time, the wood swells in the opening, helping to keep it in place despite sea conditions.
These low-tech plugs are called "traditional" for a good reason — they've withstood the test of time because they work.

Note: It is a good practice to attach the right size wood plug to each seacock in the event of disaster.