Underwater sensor networks play an essential role to enhance knowledge, usage and utilization of our aquatic environment. Its functionality leads to be widely used and gained more attention by various domains such as scientific researches, industrial, commercial, and military applications. Although there is considerable effort by underwater researchers' community to improve the performance of these networks, there are still significant challenges resulting from limited bandwidth, high propagation delay and harsh underwater environment. Security is a major performance metric which needs more focus and study in order to protect underwater sensor networks from potential threats. In this thesis, we identify vulnerabilities in existing underwater routing protocols namely, depth routing protocol. We present the attack model and propose approach to defend to secure underwater protocols against attack. We evaluate the effectiveness the proposed approach using simulation.