Replacing humans with underwater robots for accomplishing marine tasks such as oceanic supervision and undersea operations have been an endeavour from long time ago. Hence, a number of underwater robots have been developed. Among those underwater robots, developing biomimetic swimming robots has been appealing for many researchers and institutes since these robots have shown superior performance. Biomimetic swimming robots have higher swimming efficiency, manoeuvrability and noiseless performance. However, the existing biomimetic swimming robots are specialised for a single gait of locomotion like cruising, manoeuvrability and accelerating while for efficient accomplishment of marine tasks, an underwater robot needs to have multiple gaits of locomotion. In order to develop multiple-gaited swimming robots, the optimal characteristics of each gait of swimming must be combined together, whereas the combination is not usually possible. The problem needs to be addressed during the design process. Moreover, the optimality of the actuation mechanism of robots - that do not utilise any artificial muscle - could be assured using the mathematical model employed for simulation of their swimming behaviour. However, the existing models are incomplete and, accordingly, not reliable since their assumptions like the constant speed of flow around the fish robot could be used when the average speed of the flow is determined during experiment while before development of robots, the flow speed is not known. In addition to that, the simulation results must be optimised using the experimental observations in nature and analytical results while the optimisation algorithms are based on one fitness function. The aforementioned problems as well as the fabrication challenges of free-swimming biomimetic robots are addressed in a development process of multiple-gaited fish-mimetic robots introduced by the author in this thesis. This development method engages the improvement of all development steps of fish robots including design, mathematical modelling, optimisation and fabrication steps. In this thesis, the aforementioned steps are discussed and the contributions of the method for each step are introduced. As an outcome of the project, two prototypes of fish robots called UC-Ika 1 & 2 are built.