air is compressible, water is not. This effects not only the mechanics, but also the acoustics. Air is actually a much worse conductor of sound so making a quieter propeller should be easier.
The principle of using a toroidal shape to reduce creation of vortices and thus sound should hold for either medium and may even be more efficient since a vortex causes drag and they are known to improve efficiency in boats.
The principle of using a toroidal shape to reduce creation of vortices and thus sound should hold for either medium and may even be more efficient since a vortex causes drag and they are known to improve efficiency in boats.