The core of this is the creation of spatially controlled pressure/density gradients. I believe you could impart greater force on objects under water more easil

The core of this is the creation of spatially controlled pressure/density gradients. I believe you could impart greater force on objects under water more easily due to its incompressibility, but then on the other hand you face greater "inertiality" due to the density/mass of water, as well as the resistance/friction/aerodynamics. I wouldn't want to guess anything beyond that slow movements may be easier to achieve (quick small impulses) but that fast movements would be harder. The speed of sound also comes into play and which frequencies you thus have to work with, and their propagation. Sound moves faster in water (related to the incompressibility), so one wavelength corresponds to a higher frequency in water than in air. This means you might also have to be closer to the manipulated object (shorter range for the required frequencies). Another thought is that this could be used in water as a means to propel yourself without moving parts. Instead you just have surfaces acting like speakers that create pressure gradients around your submarine to create a waterflow directed backwards.