‘Virtual water’ represents water resources consumed by industrial, agricultural, and other human activities. Virtual water flow is significant for coordinating the global water balance, but most current research has focused on land. In this study, marine products or services are introduced into the research framework of virtual water, and the concept of ‘marine equivalent virtual water’ is introduced. The formulas are proposed from three aspects: food, environment, and power generation. The calculation results for China's marine equivalent virtual water content from 2006 to 2015 show a U-shaped characteristic in which different factors change in importance over time. In addition, the marine equivalent virtual water system structure is analyzed and forecasted for China's coastal areas by 2025. Through the changes in the marine equivalent virtual water system entropy, the research area is divided into three development types: equilibrium, orderly, and change. Each area can be targeted to put forward development proposals. The marine equivalent virtual water proposition quantifies the function of the ocean in the supply of freshwater resources. It provides a new perspective for relieving pressure on terrestrial water resources and is of great significance to water resource management and water policy formulation; this concept should be built upon in future research.