I want to swim because engineers have known since the 1980's that fish and marine mammals are more efficient than propellers, see here.
BUT PARTS ARE BAD! Traditional robotic fish have vertebrae, multiple connecting rods, wires, motors, joints, hydraulic pumps, etc., etc. Parts, parts, parts! It doesn't matter what they are, they are all BAD!
Traditional robotic fish have too many parts and are NOT even as efficient as propellers, let alone as efficient as real fish.
Here is a paper by the Naval Research Laboratory on this topic.
Sequence of drones
The fishBOOT -- easy, low effort, good in rough water!