With global climate change and rapid urbanization, pluvial flooding has become increasingly frequent, posing severe threats to the operational security of distribution networks. Existing studies mainly focus on post-flood load restoration after equipment has been inundated, while overlooking proactive defense strategies such as pre-drainage using mobile pumping vehicles before equipment is submerged. To address this issue, this paper proposes a coordinated pre-disaster and in-disaster optimization method for power supply assurance of distribution networks under urban flooding. First, based on a two-dimensional hydrodynamic model, an effective drainage coefficient is introduced to develop an approximate inundation depth model that incorporates the drainage effect of mobile pumping vehicles. Second, an equivalent travel time model is employed to characterize the impact of road flooding on the mobility of transportation resources, and the corresponding pre-disaster deployment and in-disaster dynamic dispatch constraints for mobile pumping vehicles and mobile energy storage vehicles are established. Subsequently, by coordinating multiple resilience measures, including mobile pumping vehicles, mobile energy storage vehicles, distributed generators, and network reconfiguration, a coordinated optimization method for power supply assurance considering dynamic evolution of inundation depth is proposed. Finally, case studies conducted on a modified IEEE 33-bus distribution system under different rainfall scenarios demonstrate that the proposed method can effectively shorten outage durations and reduce load curtailment.