Hawaii has very clean drinking water.  This is because of the artesian water beneath the islands.  When it rains here, the water soaks all the way through the ground until it reaches the seawater underneath the islands. The volcanic rock that the islands are composed of acts as a filter, so by
the time the rainwater has filtered through the ground, it is clean, drinkable water.  I think the fresh water doesn't mix with the salt water because salt water is more dense, so the fresh water floats above it.  I learned all of this in elementary school, so hopefully I haven't gotten confused.