<p>Underwater Sensor Networks (USNs) have received a lot of focus in terms of their use in environmental monitoring, ocean research and military surveillance. Current approaches to safe transmission of data in USNs are mainly based on the standard cryptographic methods, blockchain-based solutions, or energy-saving routing algorithms. Although such solutions enhance security and reliability, they tend to be poor in dealing with important underwater communication limitations including high propagation delay, low bandwidth, dynamic channel conditions and susceptibility to Byzantine attacks. Moreover, the prevailing approaches are generally isolated security mechanisms, which do not combine advanced cryptographic techniques with asynchronous consensus protocols. To overcome these constraints, this paper presents a blockchain-based framework using Asynchronous Byzantine Fault Tolerance (ABFT) along with cutting-edge cryptography, such as, Distributed Key Generation (DKG), Verifiable Secret Sharing (VSS), Multi-Party Computation (MPC), and Threshold Cryptography. The suggested solution is safe, trustworthy, and effective when data is transmitted in the underwater environment in real conditions. In order to justify the security of the suggested framework, formal and informal security analyses are taken into consideration. The framework uses established cryptographic primitives and ABFT consensus to guarantee resistance to Byzantine attacks, data corruption, and unauthorized access. Informal security analysis shows that the system meets the important security properties, such as confidentiality, integrity, authentication, and fault tolerance in adversarial conditions. The metrics that are used to carry out the performance evaluation include throughput, latency, packet loss, CPU utilization, and network lifetime. Experimental findings indicate that the suggested framework can be used to boost the security robustness and system efficiency of the current approaches.</p>