Select some component of the economy concerning water. Describe that sector, discuss price, demand, and availability. You should identify the aspects of components with the terms we have studied (such as socialism, capitalism, free market economy, command and control economy, sustenance economy, public goods, scarcity, utility). Chapters 1 and 2 in Harris should give you a foundation for this discussion, but you will need to explore your chosen water sector, and of course any other source of economic theory you choose.
For example, you could discuss the bottled water market, price and supply of water used in oil drilling, treatment of waste water, irrigation water in California or local municipal drinking water.
Remember you are writing this as an economist, so describe the system (who makes it, who buys it), discuss price (is it a variable market driven price or set by someone), availability (how scarce is it), and allocation rules (who can buy and sell).