Water purification tablets
Although they aren't 100% effective, a supply of water purification tablets would be immensely useful if you found yourself forced to drink standing water or even just water that has been sitting around too long. They don't destroy every pathogen you can find, but they will definitely increase your chances when using an unsanitary water source. Human beings can't survive more than a few days without water, but can go significantly longer without food. Making sure you have potable water will be one of your biggest concerns if you plan on getting along for long at all, and you'll be needing even more than usual because of all the exertion you'll be doing in the process of fortifying your base, killing zombies, and everything else. Tablets like these will be hard to get, but it will help your chances drastically. The most common form is iodine tablets, which kill the pathogens. Some kits also come with an additional pill that will remove the unpleasant iodine taste that is left behind. Obviously that part isn't as essential, but we all need some comforts in the apocalypse.