Dirty Dishes, You Do The Math
A dishwasher accounts for 1%-2% of the the overall water consumption of an average household of four.
This average household generates an average of 150 loads of dishes a year. Using an old standard dishwasher that consumes approximately 10 to 15 gallons of water per load this average household will use in the range of 1,500 to 2,250 gallons annually. ( This water consumption estimate does not account for water used in pre-rinsing the dishes before loading them into the machine)
Replacing the older machine with a newer efficient machine will not only save about 1000 gallons of water a year but save energy to boot.