It is entirely possible to live off the land by hunting and fishing. Food and drink are out there if you keep your eyes open.
As you wander around you’ll see rabbits, chickens, goats, boars, sheep and cows. You’ll need a hunting knife to gut the animal, once its been killed. After that you’ll get an option to collect the raw meat. It is possible to eat raw meat, but you may get infected. It is safe to build a fire and cook the meat first.
Larger pond, lakes and the ocean contain tuna, trout and seabass. Look for a fishing pole under row boats and in boat houses, in fishing boats and at certain missions. Fishing pole are equipped like a melee weapon and will give you an option to cast them when you are close to or in the water.
You can safely drink water directly from wells and fountains without getting infected. You can drink water from ponds that do have have any dead bodies nearby (look for them or listen for flies). You can fill up water bottles at any pond or lake, but you should boil the water before drinking it.