One-liter "big slam" type bottles, four of 'em, one in each side pocket, a couple in the pack. Usually only the two on the sides are full, depending on what I know about the next water. If I need to keep one cool because of sun/heat, I'll wrap it in an extra shirt that was soaked with water.

I also carry one flexible-sided Nalgene with one of those in-bottle filters in case I run into a really bad water supply. Rarely used.