Abstract—terrestrial wireless sensor networks (TWSNs) are subject to extensive research and development. Numerous applications take advantage of low-cost, small-sized, easily configurable and scalable TWSN nodes to monitor, detect, and track various environmental phenomena and events. The recent advancement in electronics and sensor miniaturization and low-power technologies enabled TWSNs to extend their reach to underwater applications. Underwater wireless sensor networks (UWSNs) can be used in many new applications. Yet, UWSNs development is dependent on a number of technological challenges that need to be overcome. In this paper, we present an overview of UWSNs, their applications, and their challenges. We also present a survey of various UWSN architectures currently used in deployed UWSN systems.