You don't necessarily need a fancy hydration system (unless that's what you prefer), but you do need some kind of water containers. I take a 1-liter and two 2.5 liter Platypus bottles, probably the lightest and most compact option. If budget is a problem, soda and gatorade bottles work fine. A lot depends on where you'll be hiking. If you're going to be in the desert or on waterless ridges, you'll need to carry plenty of water. If there are frequent water sources, one liter of water at a time is enough, but it's helpful to have more capacity in case you're camping far from water (which gives you more options for places to camp).