We develop a real-time wave information and analysis system on the basis of a data assimilation technique, the optimum interpolation method, applied for wave observation data and forecasted data. The system can provide nowcast and forecast wave information for the sea area where there is no wave observation point. Then the estimate is corrected after 12 hours with more accurate forecasted wave information. We investigate the characteristics of the nowcast wave data and confirmed that the accuracy of the data is high enough for practical applications. Splashing height and frequency of wave overtopping on coastal road is forecasted from wave information. It was shown that forecasted height and frequency from assimilated wave information is more accurate than from not assimilated one.