Earth is mostly water...why do we have droughts?

I have heard so much about droughts all over the international community, in Nevada there is supposed to be a big drought that would really cost the state billions because of some lake, or something like that which is going dry. I can't remember exactly what there main water source is called, but I did watch documentaries on it.

I want to understand how we could possibly have droughts but we have a planet full of water...what's the reason for that?

 

But why can we not just use all of the water that we have access to? I feel like the government is just letting the drought happen because they don't want to spend billions on a system that would clean the ocean water. California has all the water right at it's feet and I don't hear about systems being developed to clean all that water and make it available to people in the country who need it because of local droughts.

I just want to see them do something, it's supposed to be a problem on a national level, but we are not using our brains to fix it. Free water everywhere but we don't touch it? That's insane I think.