Oh, you'll be fine with a couple liters for drinking while you hike, and campsites can be found near water along the way.
You're going to need packs big enough for bear canisters, too. If you rent by mail from Wild Ideas instead of waiting til you get to the park to rent canisters, you'll have time at home to pack and repack them and figure that out. Here is a website on packing a can:
http://sierrawild.gov/bears/how-pack-bear-can It's about food choices really, things that stand up to being stuffed into odd shaped spaces, instead of taking things like bread, crackers, bananas, etc. Think banana chips, trail mix, rice or couscous based meals... check out trailcooking.com for lots of ideas instead of paying a ton for premade meals.
You can also rent gear instead of buying if you aren't sure you ever want to backpack again. Websites like lowergear.com can help.