Theoretical studies on the modulation of unidimensional regular waves over a flat bottom due to a current typically assign an asymmetry between the effects of opposing/following streams on the evolution of major sea variables, such as significant wave height. The latter is expected to monotonically increase with opposing streams and to decrease with following streams. To some extent, previous observations on datasets containing a few thousand of waves or over a continuous series of about a day confirm this prediction. Conversely, we analyze a multiyear dataset that deviates from the idealized conditions, featuring very broad-banded seas with high directional spread especially at high values of the ratio between tidal stream and group velocity. We characterize how the resulting asymptotic behavior of sea variables deviates from theoretical predictions in idealized conditions. In particular, we evidence a symmetrical modulation of several sea state variables respective to the direction (opposing or following) of the current with regard to wave motion.