An underwater sensor network (UWSN) is a type of three-dimensional wireless sensor network widely employed in many monitoring systems such as water quality monitoring, underwater military defense, etc. A proper secure network system can be designed to achieve the necessary security for such a network. Symmetric key encryption, which uses same secret key for both encryption and decryption purpose, is a well-established security system for wireless sensor network. Prior to deployment, the sensor nodes are pre-loaded with secret keys, which are used for encryption and decryption. The routing system that the packets use to get to the base station is another factor in the wireless sensor network’s efficiency. It is imperative to have a routing algorithm that is both efficient and portable. In this paper, a new algorithm called secure multipath angular routing protocol (SMARP) is proposed for UWSN. The algorithm uses a minimum angle-based routing system, in which packets are routed to the subsequent node based on finding a minimum angle of reference to the base station. The performance of the algorithm is found to be extremely effective and computationally inexpensive. The design, implementation, and numerous simulation results are shown in this paper.