In the nice example provided on the Feature Creation page, you could probably remove the useless line:
B) ax.add_feature(cfeature.LAND)
because you have already called
A) ax.stock_img()

As a matter of fact, B) being called after A), it should theoretically be plotted above A). I see you are using a default zorder=-1 for LAND, so it is probably below everything else (did not know you could use negative zorder values)

>>> cfeature.LAND.kwargs
{'edgecolor': 'face', 'zorder': -1, 'facecolor': array([0.9375  , 0.9375  , 0.859375])}

If I use ax.add_feature(cfeature.LAND, zorder=10), I do get land with a uniform color above the stock image on land (and the stock ocean elsewhere)