A coupled electromagnetic, fluid flow, and heat transfer numerical model was developed to investigate the microwave heating of water in a microwave oven operating at 2.45 GHz. The study compares stationary and rotating turntable conditions to evaluate the influence of mechanical motion on heating uniformity. The electric field, temperature, and velocity distributions were analyzed to understand the coupling among electromagnetic, thermal, and flow phenomena. Results show that turntable rotation significantly improves heating uniformity by averaging the absorbed microwave power and enhancing natural convection within the liquid. In the stationary condition, strong standing-wave patterns cause localized hot spots with a simulated maximum temperature of 204.96 °C, whereas rotation reduces the maximum to 171.12 °C. The average temperatures for both cases remain near 56 °C, with a root-mean-square error (RMSE) of only 0.102 °C, indicating nearly identical bulk heating behavior. Although the model neglects phase change and evaporation, the results highlight the critical role of rotation in reducing temperature gradients and preventing overheating. The findings provide insight into optimizing domestic and industrial microwave systems for safe, efficient, and uniform heating performance.