Underwater Wireless Sensor Network (UWSN) is an emerging technique for various underwater applications such as marine climate observation, pollution tracking, disaster prevention, underwater surveillance etc. And, each of these applications require Sensor Nodes (SNs) to effectively provide accurate sensed data. A node must know its own location before sending data to its neighbour. The need for location arises because the number of nodes is very large and it is not possible for the base station to find the nodes' positions, so the individual node is required to send location information along with the observed data to provide exact location to the user, which means the node must localize itself. However, due to the complex environment, it is very challenging to transmit the collected data to the base station on the surface quickly and effectively. An energy efficient routing protocol plays a vital role in data transmission. However, due to the speci characteristics of UWSNs, such as dynamic structure, narrow bandwidth, rapid energy consumption, and high latency, it is difficult to build routing protocols for UWSNs. In this article, we focus on surveying Underwater Wireless Sensor Network (UWSN) is an emerging technique for various underwater applications such as marine climate observation, pollution tracking, disaster prevention, underwater h of these applications require Sensor Nodes (SNs) to effectively provide accurate sensed data. A node must know its own location before sending data to its neighbour. The need for location arises because the number of nodes sible for the base station find the nodes' positions, so the individual node is required to send location information along with the observed data to provide exact location to the user, which means the node must localize itself. However, ex environment, it is very challenging to transmit the collected data to the base station on the surface quickly and effectively. An ficient routing protocol plays a vital role in data transmission. However, due to the specific SNs, such as dynamic structure, narrow bandwidth, rapid energy consumption, and ficult to build routing protocols for UWSNs. In this article, we focus on surveying architecture, challenges, opportunities as well as various applications in UWSN.