Blue energy, which includes rainfall, tidal current, wave, and water-flow energy, is a promising renewable resource, although its exploitation is limited by current technologies and thus remains low. This form of energy is mainly harvested by electromagnetic generators (EMGs), which generate electricity via Lorenz force-driven electron flows. Triboelectric nanogenerators (TENGs) and TENG networks exhibit superiority over EMGs in low-frequency and high-entropy energy harvesting as a new approach for blue energy harvesting. A TENG produces electrical outputs by adopting the mechanism of Maxwell’s displacement current. To date, a series of research efforts have been made to optimize the structure and performance of TENGs for effective blue energy harvesting and marine environmental applications. Despite the great progress that has been achieved in the use of TENGs in this context so far, continuous exploration is required in energy conversion, device durability, power management, and environmental applications. This review reports on advances in TENGs for blue energy harvesting and marine environmental monitoring. It introduces the theoretical foundations of TENGs and discusses advanced TENG prototypes for blue energy harvesting, including TENG structures that function in freestanding and contact-separation modes. Performance enhancement strategies for TENGs intended for blue energy harvesting are also summarized. Finally, marine environmental applications of TENGs based on blue energy harvesting are discussed.