In the OP the question referred to what container would fit best in a BOB. A good water bottle stored empty or full makes sense to me for a kit that mostly sits around. But I do use a variety of hydration bladders. I keep an empty one in my Maxpedition Gearslinger bag that I carry on business trips just in case I need to bug out and want to carry more water than my .75L Camelbak bottle.

I also recently picked up a bladder from Geigerrig. This is a pressurized bladder that uses air to force the water out thru the mouthpiece. This solved the problem I had getting my dog to drink while out walking. He didn't like the collapsible bowl nor would he use the special pet bottle with the rolling ball feed. But with the Geigerrig I just squirt water into his mouth and he laps it on down.

You can also use the stream to rinse off, irrigate a wound, or give water to someone else without them chomping on your bite valve.

It also has a fold down, flap-type, opening for easy cleaning and filling. The company web site claims you turn it inside out and run it through the dishwasher.

Just a satisfied customer.