Three-dimensionally (3D) maneuverable robotic fish are highly desirable due to their abilities to explore and survey the underwater environment. Existing depth control mechanism is focused on using compressed air or piston to generate volume change, which makes the system bulky and impractical in a small size underwater robot. In this paper, a small and compact 3D maneuverable robotic fish is developed. Instead of using a compressed air tank, the robot is equipped with an on-board water electrolyzer to generate the gases for depth change. The fabricated robotic fish shows fast diving and rising performance. A servo motor is used to generate asymmetric flapping motion on the caudal fin, which leads to a two-dimensionally (2D) planar motion. A 3D dynamic model is then derived for the fabricated robotic fish. Several open-loop control experiments have been conducted to validate the model as well as the design. It has been demonstrated in the experimental results that the robot is capable of generating 3D motion. The robot can achieve 0.13 m/s forward velocity, 30.6 degree/s turning rate, and it takes about 5.5 s to dive to 0.55 m and 10 s to rise.