Skill assessment (SA) is implemented for the KOOS real-time forecasts operated by KORDI to evaluate model performances. Based on computing SA quantities, we are able to determine if predictions are acceptable in the system. The SA we introduced provided a robust way to compare between predictions and observations. An advanced graphical plot (modified Taylor Diagram) demonstrated multiple skill quantities in a single diagram effectively. The single SA diagram for water elevations suggested that the tidal phase be fixed at Incheon and the wave effect be considered at Pohang. Additionally, the standard criteria (i.e., ‘acceptability’) for SA should be required to determine the acceptable ranges of error, target frequencies, and limiting durations for KOOS.