I use a CamelBak. I found one that is "fannypak" style. I take the bladder out, fill it with water and put it in the freezer the day before. The day of... take it out.. put it back in it's pouch and head to the river. You'll have cold water most of the day. AND... the unit serves as a wading belt and a pseudo-back belt. You can synch it up pretty tight and it will relieve your back strain.
MP