The rule below is required for all boats racing no matter what catagory. Obviously it would also be a great idea if all cruising boats complied as no matter what you are doing in your boat if you can’t plug the hole it will sink!
4.03 Soft Wood Plugs
Soft wood plugs, tapered and of the appropriate size, shall be attached or stowed adjacent to the appropriate fitting for every through-hull opening.
As you can tell from my hand in the photo this is a large bung! For boats with waterballast scoops that is a hole that could need plugging so a custom one needs to be made. I take fencing material 4 x 4′s glue them together and then turn the plugs down on a lathe.
The packs that you can buy at West Marine are pretty useless for most of my boats as only 2 or 3 work for the thru hulls and the rest are too small. It is a requirement to have them attached to the thru hulls rather than in a bag somewhere in the boat the disadvantage of this is that the wood ends up swelling up as it picks up water in the bilge. Really you want the plugs to be dry so when they are pounded into the hole they swell up when the water hits them and therefore become well wedged in the hole. So keeping a few in a ziplock bag in the dry is also a useful thing to do and will allow you to intially plug the hole with the one next to the thru hull and then change it out with a dry one after the fact.