We're on an island surrounded by water. How come we've got a water shortage?

I guess what you're talking about is why we don't use seawater to drink. We could and it's perfectly possible from a technical point of view but it's going to cost us a lot of money and a lot of environmental damage to do it. It uses a huge amount of energy. People in dry places like Las Vegas pipe in water over very long distances which costs a lot on pumping costs.