Do yourself a favor and do not add only water to the system. It will throw off your mixture of coolant and water. Buy yourself a gallon or two of straight anit-freeze, throw it in one of your storage areas and when/if it gets a little low add a 50/50 mix. If you have an extra gallon jug you can premix it and have it ready for when it's needed. Simply pour half the coolant into the empty container and then fill both containers the rest of the way with water. Make sure you use a permanent marker to write "50/50" or some other way to remember that the anti-freeze has been mixed already.
Doug