Digital twins are virtual models of physical systems used in various industries for monitoring, control, and prediction. Recently, digital twins of the ocean have been developed to simulate and predict ocean dynamics such as currents, temperature, and sea level. For example, they can help predict conditions that support decision-making in offshore energy, fisheries, and shipping. Ocean digital twins have the potential to transform fields such as marine biology, ocean engineering, and climate prediction by providing real-time insights into ocean behavior. They can revolutionize research, support sustainable development, and contribute to the blue economy. The user-friendly interfaces of these systems will allow researchers and the public to access data, visualize scenarios, and interact with simulations. This presentation will review advances in ocean modeling, data assimilation, artificial intelligence/machine learning (AI/ML), and user interface design under the UN Decade of Ocean Science, with a particular focus on how these digital twins are transforming both research and operational frameworks in marine science and industry worldwide. The development of such tools for coastal regions of Japan will also be discussed, with possible outreach activities to other parts of the world.