All the hype about the fuel cell (which wasn't even in the fish) obscured the really amazing thing: They replaced a conventional motor with a magnet and coil arrangement that mimics the tail motion of a real fish and it made it 100 times more efficient. Nice.
That's an amazing video, how does it change it's vertical angle, I can't seem to catch how that works, but it is very pronounced, especially near the end of the video?
It is able to change its center of gravity (it's mentioned near the beginning of the clip).