Underwater acoustic metamaterials have emerged as a powerful platform for manipulating sound propagation in aqueous environments through carefully engineered metallic structures. This chapter explores the design, simulation, and experimental validation of five distinct acoustic asymmetric transmission (AAT) systems based on underwater metamaterials. The first system employs a thin brass plate with a single-sided periodic grating, demonstrating tunable AAT effects below 1500 kHz through unit cell optimization. In addition, an enhanced AAT configuration combines brass cylinder gratings with multi-layer plates, achieving remarkable performance with 0.97 transmittance and a 104 rectifying ratio via the excitation of leaky A₀ mode. Further investigations reveal how grating period and plate thickness precisely control transmission bands and output angles by manipulating diffraction orders and Lamb wave modes. A multi-band AAT system is then developed using dual gratings with different periods, producing four working bands below 1500 kHz through complex mode interactions. Most notably, an ultra-broadband AAT device incorporating a brass plate and right triangle reflector achieves an unprecedented 2000 kHz bandwidth at 48° incidence by exploiting angular-dependent A₀ mode excitation. These designs not only advance fundamental understanding of underwater wave manipulation but also offer practical solutions for medical ultrasound imaging, sonar detection systems, and next-generation acoustic components, demonstrating the potential of underwater metamaterials in both scientific and engineering applications.