Custom duties generally refer to taxes imposed by a government on the import or export of goods, including food and water. They are a part of international trade regulations and are intended to raise revenue for the government and protect local industries from foreign competition.
However, if you're asking about defining "custom duties" in a more metaphorical or symbolic sense as food and water, that could imply that the basic functions of duty and responsibility are essential for sustenance, much like food and water are essential for physical survival. In this context, custom duties could represent the obligations or responsibilities that nourish society, helping maintain order and growth.
If you meant something else or have a specific context in mind, please provide more details!