In the application of underwater creature study, comparing with propeller-powered ROVs and servo motor actuated robotic fish, novel biomimetic fish robot designs with soft actuation structure could interact with aquatic creatures closely and record authentic habitats and behaviours. This final project report presents the detailed design process of a hydraulic soft actuator powered robotic fish for aquatic creature study capable of swimming along the 3D trajectory. The robotic fish is designed based on the analysis of the pro and cons of existing designs. Except for the mechanical and electronic designs and manufacturing method of crucial components, a simplified open-loop control algorithm was designed to check the functionality of the application board and microcontroller in the Proteus simulation environment. As the key component of the robotic fish, Finite Element Method (FEM) simulations were conducted to visualise the soft actuator's deformation under different pressure to validate the design. Computational Fluid Dynamics (CFD) simulations were also conducted to improve the hydrodynamic efficiency of the shape of robotic fish. Although physical manufacturing is impossible due to the pandemic, the simulations show overall good performance in terms of control, actuation, and hydrodynamic efficiency.