This paper aims to investigate the seasonal fluctuations in recreational boating demand in the Croatian Nautical Port System from 1998 to 2018. The number of vessels in transit is used as an organizational performance indicator of seasonal fluctuations. This research paper proposes a three-parameter model for the classification and sub-classification of each month during a nautical season by the variations in demand. To develop the model, the authors propose statistical measures: an average, a spread ratio coefficient, and a correlation coefficient. The proposed Three-Parameter Classification Model enables more efficient market demand analysis in the long run. The results of the research show differences in demand intensity across a nautical year and conform to a seasonal character of nautical sport and recreation in the Croatian Nautical Port System (CNPS). The results are confirmed by conducting ANOVA analysis on variables (p < 0.00001), and t-test statistics on classification clusters (p < 0.016).