HydroTWIN (HT) is an underwater acoustic self-contained digital twin solution using numerous Industry 4.0 technologies, including IoT, Numerical Modelling, High-Performance-Computing, modern Data Visualization and Artificial Intelligence. The solution permits to monitor, detect, geotag and classify underwater anthropogenic and natural noise sources, using passive energy efficient acoustic systems. It is intended to be hardware and maritime platform agnostic, and to be used mutually for Ocean Sustainability purposes (monitor and mitigate noise environmental impacts) and Security and Defence applications (detect, identify, and act on any maritime threat).