500 ml. You're never going to fit a nalgene-sized bottle in that mesh pocket.

Those pockets are ya, pretty useless for water bottles. I've found them useful for stuffing light rain gear in though. Really quick to grab and you do not have to open your pack. I've also used them for things like rope, tarps, etc. Pretty much I've used them for rain stuff, quick to grab and stuff doesn't get when after you use it. Drying gear out is ideal, but not always really in the cards. There is a water bottle holder which will house a 1lt in the right hand side pocket (if you are wearing the pack) and I've stuff an aditional 500ml in the pocket with it before. You cannot really access the bottle while it is on your back though so I'd say spend the 12 bucks and get the 3lt H20 hydration system that fits inside the pack and feeds to your sholder. You can drink without breaking a stride. ALWAYS TAKE A BOTTLE, the water pack is great but do not replace the bottle. If you get stranded a solid bottle can become a very useful tool.

Remember that if the pack is stuffed solid in the bottom, putting the water bottle in first won't necessarily help. It might stay in for a while, but as soon as the bottle has a chance to wiggle around or if you do any bushwhacking, it may pop out. I have lost a couple of water bottles this way. I still like the side pockets for accessibility but now I try to clip everything with a carabiner to a strap if it sticks out of the pocket at all.