Hydration bladders are my favorite way to carry water. I use Platypus products. My regular pack has a sleeve that will handle a 3 liter bladder, which is a lot of water really-- I hike in the Western Cascades, where there are many water sources. I treat it with Aqua Mira liquid chlorine dioxide and tape Micro Pur tablets to the bladder as a backup. I also work an empty 1 liter Platypus into my PSK for backup. It too has 4 Micro Pur tablets (in foil packs) taped to it.
I think hydration packs are the way to go for day hikes. Get one with some extra storage and a "stuff-it" area where you can stash rain gear or a fleece. The small hydration packs make a great base for a PSK. Being able to drink on the fly is great and you lessen the chance of getting dehydrated. Rather than having to stop, you just sip as you go.
Someone mentioned bottled water. Using recycled bottled water containers has been an ultralight hiking choice for years. It is the cheapest, lightest way to carry water. It is surprising how tough they are too. Nalgene and CamelBak hard containers are strong, but they are fairly expensive and they weigh about 6 ounces each. I can make a nice PSK that weighs 6 ounces, let alone the 12 ounces eaten up with carrying two.
I've been using stainless steel water bottles for EDC and I like them. The Kleen Kanteen bottles are everywhere now and work well. H2Go makes some stainless bottles that are thinner and lighter than the Kleen Kanteen rigs. They are usually sold for promotional stuff and are harder to find. The real benefit to the stainless bottles is that you can boil water in them in a pinch and avoid having to work a pot into your kit. The Goyot wide mouth bottle would make a good PSK container, but I would like to see something like the Kleen Kanteen threads on a wide mouth steel bottle for a better boiler and PSK container. Thinner walls wouldn't bother me and a titanium wide mouth bottle would be perfect, but I don't want to think about the cost. BTW, the stainless pots made to fit the Nalgene bottles work just fine on the 40oz Kleen Kanteen bottles.
If you just want to carry a water bottle, there are all kinds of slings and pouches around. I recommend trying to walk with a one liter bottle hanging off your belt before relying on it-- I don't care much for it. The military canteens sit a little closer and are made for carry on a big belt and are a little easier to live with-- with the proper belt. If I'm just going for a walk, a sling style carrier worn bandolier style is more comfortable for me.