Protecting marine environment is an important and urgent task to maintain marine ecological balance. As a promising blue energy harvesting technology, triboelectric nanogenerator (TENG) can provide electrical energy for marine environmental protection. Previous reported swing-structured TENGs have advantages of low driving force in low-frequency water waves and strong durability, but the space utilization and volume power density are relatively low. Here, a multilayered swing-structured TENG (MS-TENG) with low wear is designed to increase the space utilization rate, and thereby enhance the output performance. The MS-TENG generates the highest peak power density of 15.18 W m−3 Hz−1 and average power density of 3.56 W m−3 Hz−1 under water wave triggering at 0.8 Hz. By coupling the charge pumping and non-contact mode of main TENGs, the MS-TENG achieves strong performance stability, exhibiting little attenuation of electric output after 240 000 cycles. Furthermore, the MS-TENG has successfully powered a water quality detector for self-powered ballast water quality monitoring, and an application is presented about electrical dehydration of water-in-oil emulsions driven by the MS-TENG with a dehydration rate of 99.6%. This work promotes the development of high power density MS-TENG and demonstrates its application potential for self-powered marine environmental protection.