In bio-inspired robotics, there is significant interest in using new materials with diverse mechanical properties to enhance the kinematic and dynamic performance of mechanisms, such as flexible and soft materials. These materials not only reduce friction forces but also enable the replication of natural animal behaviors, simplifying the mechanical design process. However, in hostile environments like underwater, soft and flexible materials have not been extensively tested under varying conditions of depth, temperature, and pressure. This article presents the mechanical design of a bio-inspired robotic fin. The mechanism consists on a cable-actuated flexible joint, integrated into a spatial parallel mechanism with 2 degrees of freedom (DoF), designed to replicate the natural movement of Mobula Alfredi, commonly known as the Reef Manta Ray. Finite element analysis and dynamic simulations are conducted to evaluate the materials and mechanism's behavior. Additionally, a prototype was developed using modern fabrication technologies and flexible materials. The results demonstrate that the proposed cable-driven flexible joint meets both the mechanical and kinematic requirements, even in calm underwater conditions.