Body of water #18 refers to a specific body of water that is being mentioned. In this case, the answer is "Persian Gulf" which is a well-known body of water located in Western Asia. It is an extension of the Arabian Sea and is bordered by several countries including Iran, Iraq, Kuwait, Saudi Arabia, Bahrain, Qatar, and the United Arab Emirates. The Persian Gulf is significant for its strategic location, rich oil reserves, and its importance in international trade and shipping.