These things are great for fly fishers! I've been using the large one for a few years, but now they are out in a smaller size that is perfect for fishing, since we are always near water. The small ones have a flap on them you can use to attach the bottle with a carabiener to your vest or wading belt, no holster required.

What makes them so great? The bottle has a built-in water purification system. You can carry them empty and, whenever you are thirsty, just dip them in the water around you to fill up the bottle with cold water that is perfectly safe to drink. Then you squeeze the bottle to force water through the filters and out the straw-like dispenser into your mouth.

Cold, safe water whenever you want it and little weight to carry. They cost about $30 and are good for 26 gallons of water before the filter needs to be replaced.