Interesting subject. I believe that magnetohydrodynamic propulsion has failed because everybody used DC.
But if you reverse the current and ALSO the magnetic field, the Lorentz force does not change. So maybe using High Voltage, low intensity AC with electromagnets, the contraption might work.
There is another way of propulsion. Let me call it the Groundworm propulsion. You have an elongated hull with a mouth in its front end that continuously regurgitates an elastic skin, and another orifice at the rear end that keeps swallowing this skin. The skin runs along the hull adapted to it through a low viscosity lubricant. The relative velocity of this skin and the surrounding water is zero or nearly zero, so there is no friction with the water and due to this no turbulence and no noise at all.
I forgot, the skin runs back inside a pipe in the submarine. This is not a Sci-Fi idea but a useless one. Because you cannot get out or enter the submarine, unless you break the skin.
But I believe that what the writer was trying to explain is a "real" caterpillar drive as in the Caterpillar tractors.
You install half of the drive inside and the bottom half outside the hull. .And, somehow you seal it.
Then you provide this mechanism with large paddles, and even with an imaginative design you make these paddles to come in and out in a perpendicular motion, to minimize the relative velocity Paddle/Water which is what produces the small vortices where the noise comes from.
The larger the paddles, the smaller their velocity differential with surrounding water and so likewise the smaller the vortices.
This was probably the propulsion in the mind of the writer.