Fish are more efficient than propellers
Scientists have known since the 1980's that fish and marine mammals are more efficient than propellers. This is a graph comparing efficiency of marine mammals to a large, slow moving, very efficient propeller.
In addition, propellers have a narrow zone of peak efficiency at a target speed. Go faster or slower and efficiency drops in a non-linear manner. The efficiency curve of a propeller can be made flatter (more like a fish's), by using variable pitch propeller blades. However, this requires more drive-train components. Due to increased friction from the additional drive-train components , the peak efficiency for variable pitch propellers drops, even as the curve becomes flatter.
The efficiency of most propellers on most boats is MUCH lower than as shown in this graph, closer to the 0.2 level. Most boats have small, fast moving propellers which are VERY inefficient, and most boats also have a driveshaft that is not straight. Every bend in the driveshaft reduces efficiency further. This is a graph of a highly optimized propeller/drivetrain/boat hull.
This graph is from, "Hydrodynamic flow control in marine mammals," by Frank E. Fish, Laurens E. Howle, and Mark M. Murray, From the symposium ‘‘Going with the Flow: Ecomorphological Variation across Aquatic Flow Regimes’’ presented at the annual meeting of the Society for Integrative and Comparative Biology, January 2–6, 2008, at San Antonio, Texas, Integrative and Comparative Biology, volume 48, number 6, pp. 788–800
Okay, so REAL fish are more efficient than propellers, but in a ROBOTIC fish, how do you connect the motor to the fin without a lot of parts? Robotic fish have all been less efficient than propellers, due to the number of parts.
At the end of the day, robotic fish compete with subsea watercraft like the Remus that and are incredibly simple and efficient. Craft like the Remus have a large propeller and a straight driveshaft connecting to an electric motor. There is a bearing for the driveshaft. Ther is also a driveshaft seal, which keeps water from coming up the driveshaft. But this driveshaft seal is a really bad part. Leakage up the driveshaft limits how deep the craft can go. The driveshaft seal can be made to seal better, but this increases friction and reduces deployment time for Remus and similar watercraft. The problem of the driveshaft seal is actually quite severe.