Oahu's water system comes from rainfall in mountain watersheds which percolates through miles of lava rock into groundwater aquifers that are tapped by wells for your water supply. If rainfall is decreased due to cycles of drought, and pumping for water consumption is high during dry periods it can begin to deplete our available water supply. Even though we are surrounded by water, the ocean is salty. To be able to drink it we would need to remove the salt, or desalinate it. This technology has been experimented with in Hawaii but it is still experimental and costly.