So only water vapor has a significant effect on air density when temperature and pressure are held constant. Thus, if we increase the proportion of water vapor molecules in the air mixture while keeping temperature and pressure constant, we lower the density. But not by much. Thus, one could expect to drive a golf ball only a few yards farther on a very humid, hot summer day than on a very dry, hot summer day. With the effect of backspin added, the gain is even less, perhaps only half as much.
Temperature has a much greater effect on air density than humidity.
Thus, on a given golf course, one should expect to drive a golf ball some tens of yards farther on a hot day than on a chilly day.