Analogy of Shawyer's EMDrive should work also on water surface. You can imagine the resonator like the boat without bottom (bottomless wooden washtub so to say), which is floating at the water surface. From this perspective the EMDrive behaves like the conical barrier, floating at the water surface. Try to imagine, we are doing ripples & splashes inside this barrier, which are bouncing back and forth, but because they cannot leave the barrier, they wouldn't spread into outside. If we would neglect the (existence of) underwater, then the floating barrier couldn't propagate in any direction in similar way, like the classical physics predicts for EMDrive in vacuum. But the surface ripples also induce an underwater sound waves, which can escape beneath the barrier, and because it's wider at one end, the sound wave pressure will push it into reactive motion in opposite direction. The underwater sound waves also manifest itself like tiny turbulences at the water surface, which are speeding up the surface wave spreading.
According to this analogy what propels the EMDrive forward is the stream of scalar waves/dark matter particles, generated with magnetic turbulence from vacuum inside the resonator. It's invisible noise detectable only be SQUIDs and similar magnetic field detectors, but it still curves the space-time and as such it exhibits inertial mass. It can be therefore interpreted like the sparse worm hole and also volume area of negative energy in warp drive model of EMDrive. It generates mass density gradient (actually artificial gravity field) inside the resonator, which would propel the EMDrive forward. The scalar waves do behave like bubbles or turbulence of vacuum and the light should propagate faster in this environment, which can be detected by fringe shift of White–Juday interferometer.
In this connection it may be significant that, if the photons would be massless, they should always propagate with speed of light, isn't it true? But the experiments show us, it's not always true - the photons can be slowed down by their polarization (1, 2). The reason of this behavior is, the photons can gain the mass from their polarization (spin) - more info. Intuitively speaking, the energy introduced into their spin angular momentum travels together with photon and it doesn't depend on its wavelength with compare to intrinsic momentum of photons. So it behaves like the energy gaining rest mass of photons and it gives the photons ability to annihilate it during mutual interaction under violation of Newtonian laws.
See also: Photon mass drag and the momentum of light in a medium, Is the frustum EM Drive4 decelerating light for propellantless propulsion?
