Wireless communication across the water-air interface is an ongoing challenge since no single type of wireless signals can operate well across different media. Underwater acoustic signals, noticeably, can cause tiny displacements on the water surface. These displacements contain the information of sound sources and therefore can be extracted by terahertz radar. This paper at first presents both a point target model for narrow-beam terahertz radar and a face model for wide-beam terahertz radar. The face model is established according to the facet method. The radar echo is then simulated according to the different models and the time-frequency analysis of the surface vibration is carried out. The effective water-surface vibration model will lay a foundation for information extraction of underwater sound sources and eventually for communication.