Arguing that it's a waste of water, however, isn't one of them.
I've seen a few different people criticize the trend for wasting perfectly good, clean water. That may be true, but when you look at the amount of water wasted in the context of the staggeringly huge amounts of water we use to produce food — especially animal products — the ice bucket challenge is truly a drop in the bucket (sorry).
These numbers are based off an excellent chart on water use made by Eric Holthaus at Slate, and the assumption that the average bucket of ice and water contains five gallons. But even if the bucket contained 50, it'd still waste less water than is used to produce a single egg — and about a thirtieth as much as is used to produce a pound of beef.
The reason that animal products require so much water is that you have to take into account all the water used to grow the crops to feed the animals. For an average cow raised in an American feedlot, it takes somewhere between 5.3 and 7 pounds of corn to produce a single pound of beef.
Even if every American did the ice bucket challenge, we wouldn't drain our groundwater supplies. It's our agricultural methods and eating habits (and, perhaps, global warming) that are already causing our wells to run dry.