The whole idea of the Manta Drive was inspired by a visit to an aquarium where members of the public got a chance to pilot small ROVs round an obstacle course. I got my first look at the ROVs and realised two things:
There were a lot of places for the water to get to the insides of the ROVs
The ROVs didn't look right - they were just boxes, and didn't look designed to swim. They lacked the elegance I associate with swimming animals.
Later cogitation also came to consider power - the high-revolution impellers used by the ROVs struck me as power-hungry. I may be wrong, and I have not tested the power consumption of the Manta Drive, but this is a secondary consideration.
As I wandered the aquarium, the ROVs played on my mind, and I found myself comparing them to each animal I saw. How did they compare? Could the animal's swimming motion be replicated elegantly, in a way that maintained hull integrity*?
Looking at fish like rays, sea cucumbers and stonefish, I realised that the most elegant propulsion method was the waving fin.
I also realised something important - fish don't leak. A rotating shaft needs to pierce the hull completely, working through a hole in the hull. On the other hand, a reciprocating motion (up-and-down) could work through a flexible, waterproof membrane which did could be fixed firmly around any moving parts without ripping.
I further realised that flexible membranes could wear out, but magnets don't, and magnets can act through any non-magnetic materials without restriction. Make the hull rigid, but non-magnetic, and the risk of leaks due to the drive system are completely eliminated.
* Oh, I went all Star Trek for a second there!