Exploring the vast expanse of oceans, Earth’s largest realm and a vital resource repository, has intrigued humanity, transcending disciplinary boundaries. Recent breakthroughs in science and technology have propelled us from surface endeavors into the depths of the underwater world. However, a host of challenges emerges beneath the waves, encompassing energy constraints, limited sensing capabilities, and communication obstacles for submerged equipment. These challenges, which resonate across diverse fields, demand innovative solutions to advance scientific pursuits and practical applications. Triboelectric nanogenerators (TENGs), pioneered by Zhong Lin Wang in 2012, represent a revolutionary leap in energy harvesting and self-powered sensing technologies. These devices have fostered exploration and innovation in various domains, including marine science and technology. In response to the unique challenges posed by the underwater environment, a specialized application of TENGs has emerged: underwater TENGs. These innovative devices hold the potential to transform underwater exploration by increasing energy harvesting, expanding sensing capabilities, and enhancing communication systems. This comprehensive review aims to demystify the domain of underwater TENG technology, delving into the theories, design strategies, and recent breakthroughs that underpin this emerging field. This review also contemplates the future trajectories of underwater TENGs, envisioning their transformative role in elevating underwater research and technology.