Abstract: Effectively controlling ocean wave propagation, reducing wave-induced loads, and improving the safety and service performance of floating marine structures remain critical scientific challenges in the field of ocean engineering. In particular, the control of ultra-low-frequency, high-energy long waves in deep and far seas is especially difficult. To address this bottleneck, a bionic seagrass-like flexible water-wave metastructure is proposed in this paper, inspired by the efficient wave attenuation performance of marine flexible vegetation and drawing upon the local resonance mechanism. The metastructure consists of a periodic array of flexible beams deployed at a prescribed water depth, each serving as a local resonator. Based on the linear wave theory, a fluid-structure interaction dynamic model is established to characterize the interaction between the vibrations of the bionic flexible water-wave metastructure and the water wave motions. A semi-analytical hybrid analysis method combining eigenfunction expansion matching and modal expansion is developed, in which velocity auxiliary functions are introduced to resolve the computational singularity at the tip of thin-walled structures and to accelerate numerical convergence. Wave flume experiments are also conducted to validate the theoretical model. Convergence analysis verifies the accuracy of the numerical method, and the calculated transmission coefficients are found to be in good agreement with the energy conservation law and the Haskind relation. Numerical simulation results demonstrate that the flexible water-wave metastructure can open a pronounced local resonance bandgap when the incident wave frequency approaches the natural frequency of the local resonators, accompanied by wave energy concentration near the resonator. The bandgap position can be flexibly tuned by adjusting geometric parameters such as the beam thickness, and it exhibits coupling effects with Bragg resonance bandgap. Experimental results are consistent with theoretical predictions, confirming the effectiveness of the flexible metastructure in low-frequency wave attenuation. Furthermore, the experiments reveal that during local resonance, distinct vortex structures form around the resonator tips, accompanied by significant energy dissipation. By tuning the stiffness of the flexible beam resonators, target low-frequency bandgaps can be effectively designed and regulated. The proposed metastructure provides a novel and promising approach for low-frequency wave attenuation and the protection of marine structures.