The Forespar Sta-Plug form fitting emergency plug is used to stop water from entering your boat via a hole caused by maintenance failure (broken thru hull), or an accident (hull breach).
Even a small hole can sink your boat very quickly. For example, a 2 inch 1 foot below the waterline would fill a 55 gallon drum in 40 seconds. Don't try to rely on a bilge pump for emergency situations like this.
The Sta-Plug is designed to seal most seacocks, valves, thru hull fittings and hoses to stay afloat until further help arrives.
Needed to replace a below the water ball valve in my new to me boat, that was made of the wrong material and had seized open. Shoved the Sta-Plug in the thru-hull from under the boat. Went inside, removed the hose, saw no water coming in, removed the old ball valve and screwed on a proper marelon valve. Literally only a couple of drops of water came in. Easy and done.