Wait, so let me get this right: Instead of donating money, you dump water on your head?
Brilliant.
This is just another way for people to pat themselves on the back, so they can feel like they are making a difference.
This is much different than actually rolling up your sleeves, getting "dirty" and advocating for a cause. There are those rare individuals who have changed laws, policies, donated money, or whatever and have changed the world. Imagine if these people just dumped ice water on their head, instead.
If you took every person who dumped ice water on their head and really made them advocate for a cause (ending child abuse, homelessness, etc), the results would be staggering. Instead, they just fill up a bucket of water and pour it on themselves.