Water and Agriculture Tax Reform Act of 201 8 This bill amends the Internal Revenue Code to permit tax-exempt mutual ditch or irrigation companies to earn income from dispositions of certain real property and stock interests without affecting their tax-exempt status, but requires that such income be used to pay the costs of operations, maintenance, and capital improvements of such a company. The bill also establishes a rule regarding the organizational governance of mutual ditch or irrigation companies. Where state law provides that such a company may be organized in a manner that permits voting on a basis that is pro rata to share ownership on corporate governance matters, the tax-exempt status of the mutual ditch or irrigation company must be determined without taking into account whether its member shareholders have one vote on corporate governance matters per share held in the corporation.