This paper provides an overview of underwater wireless communication modems, mainly focusing on optical, acoustic, electromagnetic and hybrid techniques. Underwater optical modems uses a light signal for transmission data, which offers high data rates but are limited by the absorption and scattering in water, hence, it is suitable only for short-range and clear-water applications. While, the acoustic modems supports for longer distances, but it face challenges like low bandwidth and high latency due to the characteristics of sound propagation in water. Whereas the hybrid modems are aimed to achieve the strengths of both optical and acoustic techniques, to achieve high transmission range and data rate by dynamically switching between the modems. This paper presents the design challenges such as energy efficiency, miniaturization and environmental adaptability. In addition to advancements improved in recent modulation techniques like adaptive algorithms and multi-modal communication protocols. This study identifies critical obstacles, including power constraints, environmental variability, and limited range in hybrid systems, while proposing potential solutions to enhance underwater communication for applications like oceanographic research, military operations, and environmental monitoring.