You can survive for weeks without food, but you can only survive a few short days without water.
Dehydration is a major issue now in Haiti, but on top of that there is a need for clean water. With infrastructure ruined a lot of the water that comes from pre-existant sources is now contaminated. So the option for many is either to not take the water and dehydrate or drink contaminated water and risk disease.
The DEC appeal lists some of the amounts that you may be able to contribute and what difference it makes.
If you haven't already please give.