Blue photodetectors are critical components for underwater wireless optical communication, high-resolution bioimaging, and environmental monitoring. However, there remains a need for more practical and energy-saving devices with facile synthesis and stable materials. Herein, we developed a blue photodetector featuring a unilateral Schottky junction on multilayer WS₂ decorated with Ag nanoparticles (Ag NPs). The synergistic cooperation of high-energy exciton from WS₂ and localized surface plasmon resonance (LSPR) induced by Ag NPs enhance responsivity and external quantum efficiency up to 244 mA/W and 74.7% under 405 nm with zero external bias, respectively. Ultrafast transient absorption spectroscopy directly reveals the LSPR effect significantly enhances the generation of high-energy excitons, providing a microscopic mechanism for the improvement. Critically, this device enables 128 × 128 resolution blue-light single-pixel imaging in both clear and turbid water, achieving a high peak signal-to-noise ratio (PSNR) of 25.5 dB. This work provides a new strategy for developing the self-powered underwater optical communication and high-visibility imaging technologies.