Water is the most important of the essentials. You should always leave for an outing with a full water bottle. On backpacking trips be sure to have a two-liter water bottle, so you can't get dehydrated. If you are going to a place where there isn't going to be a spicket around to fill your bottle, bring some iodine tablets or a water filtration kit. If you don't have access to those, always boil your water before drinking when it is from a natural source.