Water loads are widely used in high power systems of microwave heating to absorb energy non-absorbed by the processed material. In the project, the load designed for a 915 MHz 75 kW transmitter (a segment of a rectangular waveguide containing a conical cavity filled with water) is simulated with the use of QuickWave-3D, electromagnetic modeling software based on conformal FDTD method. Coupling of the system and patterns of power dissipated in the water cone are analyzed as functions of geometrical parameters and temperature of water. Obtained data should be used to generate information on any inherent power limitation in these loads and to design similar loads with good operating characteristics for other transmitters.