Thought it might be interesting to hear what gizmos, gadgets and witchcraft we all keep aboard to deal with a potential leak at sea. For starters, I insure each of my seacocks are fully functional, as I close nearly all of them each time we leave the boat. Reaching down to quickly turn a handle and stop a leak from a ruptured hose to find the handle won't move would be a panic and shameful.
I also have the quintessential set of dowel plugs and hammer. Several sets, actually. I have not gone to the effort of attaching the proper one to every single thru hull, as is suggested. They should also be in a water resistant baggie, as they are designed to absorb some water and swell, so you want them to start dry. However, I find the odds of having a perfectly round hole that could be repaired with a dowel to be the least likely event.
I've collected several items from boat shows. Here are a couple of others I have aboard.
Stay Afloat. This stuff seems pretty cool and will work on the more likely odd shaped leak.
Also have these aboard, which will work in odd shaped holes too: