Dialy mean sea level time series recorded in Naples (1959--1976) and the joint variation of sea level, atmospheric pressure, and wind together with the values of water density and marine currents are analyzed. The daily mean sea level series adjusted according to the hydrostatic hypothesis shows a pronounced seasonal cycle, principally explained by the seasonal cycle of water density. The cross-spectral analysis shows surprisingly small barometric factor values for periods shorter than a week, explained mostly by the influence of local wind. An autoregressive model for mean sea level forecasting has been set up and satisfactorily applied to practical problems such as the discharge of rivers and the continuous surveying of movement of the ground from a tide gage alone.