Fecal pollution caused by combined sewer overflows has been observed at urban waterfronts after rainfall and results in fecal pollution levels exceeding bathing water quality standards. To guarantee the swimmablility of urban waterfronts, it is necessary to predict the situation of fecal contamination to ensure safe bathing. Therefore, we aimed to develop a water-quality forecasting system for the beach in Odaiba Marine Park using Escherichia coli as a fecal indictor to determine whether people can submerge their faces in the seawater on the day of bathing. First, rainfall events in the 23 cities of Tokyo over the past 10 years were categorized by cluster analysis, considering their temporospatial distribution characteristics. Then, E. coli concentration changes in the park were calculated using a 3D hydrodynamic and water-quality model for each categorized rainfall group. Using the calculation results for various tidal conditions, we constructed a database of E. coli concentration changes. It was confirmed that the E. coli concentration predicted using the database for a corresponding rainfall group agrees with the concentration peak calculated using the model within a safe margin. Using the constructed database, we carried out a trial operation of the water-quality forecasting system for the beach in 2018 and verified its effectiveness.