Abstract: High dams and large reservoirs are critical infrastructure that support water safety and security systems. Their safe and stable operation plays a vital role in ensuring sustainable socio-economic development and public safety. However, certain defects develop beneath the water surface, rendering them difficult to detect and potentially threatening the long-term safety and reliability of these structures. This study reviews domestic and international cases of underwater distress in high dams and systematically summarizes existing defect detection methods and equipment, including optical and acoustic imaging techniques. It also highlights recent advances in underwater defect detection, damage identification, construction within deep-water digital twin frameworks, and structural health diagnosis and assessment. Key scientific challenges are identified, such as acoustic-optical fusion imaging under multi-field perturbations, high-resolution detection of deep-water defects, multi-source data fusion-based characterization, error propagation between underwater and land-based digital twin systems, and rapid defect diagnosis. The aim is to provide technological support for precise underwater detection and enhance safety assessment of high dams.