Bear, the pouch itself holds at best 8-10 oz. of fluid, so it isn't a good primary choice for water. Useful space is around 3.5" by 7". How thick depends on your comfort level - I found it easy to over-do in my excitement to put everything I own in it. But it easily has room for a medium Reynolds Oven Bag, which is my current primary water carrier. I hang my whistle, Photon, and compass on the lanyard.