In Metal Gear Survive, getting food and clean water can be a major pain in the ass. Here is something to lessen the sting a bit.
Obtaining clean water
After you reach Chapter 7 in the single player campaign, you'll unlock the Campfire and upgraded Hanging Pot. Build both for an easy source of clean water. Make a canteen to store your clean water and keep an eye out for clean bottles of water in surrounding areas.
Getting yourself some food
At the main base, there are potatoes in a variety of locations. Make your way down the hill to the small cluster of buildings and search by the burn pit. Also, look around the wrecked copter to find some potatoes for your garden. Corn is arguably better than potatoes, and can be found to the east of your basecamp. Although gardens are good to have, growing is not the best way to stay fed, since it takes a couple hours to grow, so you'll need to keep hunting for game and food items. A good pair of headphones can help when listening for small animals, since they squeak and make other small noises. Make sure you search all over, since you can overhunt an area easily.