Earth is solitary among the rocky planets revolving in the solar system. It is the only planet that is 3/4 covered with oceans of liquid water. In today's era, wireless sensor network is used in almost places and also is an interesting topic for researchers too. But with the advancement in technology, a network similar to wireless sensor network can be deployed under the water with acoustic signals for data transmission. Because of harsh environment and some more challenges, underwater communication becomes tricky. Underwater wireless sensor networks (UWSNs) is an advanced technology for underwater explorations and have demonstrated their quality in underwater applications. UWSN is a blend of wireless technology with sensor technology that has some extraordinary features like smart sensing, intelligent computing, and communication capabilities. Further, to maintain the secure communication underwater, it becomes more and more tricky. This chapter presents an expansive analysis of challenges, attacks, and security techniques of UWSNs.