Answer: A hydraulic propulsion system.
Created in the Computer Science and Artificial Intelligence Laboratory (CSAIL) at the Massachusetts Institute of Technology, the robot is called SoFi, which stands for soft robotic fish. It is the most versatile fish robot made for scientific use, according to its creators.
SoFi’s outer shell is made of 3-D printed plastic, meaning it would be easy and inexpensive to recreate. It responds to commands from an SNES controller so long as it is within sight of the operator while continually swimming forward in semi-autonomous mode, meaning it doesn’t need to be constantly controlled.
Most notable, however, is its tail. Housed within this appendage are a pair of chambers and a hydraulic actuator that pumps water through them, creating a lifelike movement of the tail that propels SoFi forward. This system is safer, quieter and more realistic than that of most other robotic fish.
See SoFi in action in the video below.