This paper presents a control architecture for the underwater locomotion control of a biomimetic amphibious robot with multi-mobility mechanism. In view of both hydrodynamic problem and engineering approach, we develop a robotic prototype capable of multi-mode motion. A robust gait control for steady swimming using the central pattern generator (CPG) is proposed and has been successfully applied to the robot. The CPG can produce coordinated patterns of rhythmic activity while being simply modulated by control parameters including input drive, frequency, amplitude, threshold, etc., which will be suitable for manually interactive modulation. Using the CPG model, the robot is capable of performing and switching between various locomotion modes such as swimming forwards and backwards, turning and pitching, with the speed, direction and gait types modulated accordingly. A test-bed is provided and results are presented demonstrating interesting properties of the CPG-based control approach and feasibility of the CPG control for efficient propulsion.