On a slightly-longer-than-necessary-immediate-family-only-two-week trip to the Adirondack Mountains over the summer past, I learned how to waterski on one ski. Once up on two skis, the forward velocity of the boat makes the water 'act' as it would if you had fallen on it from a very high position: it 'hardens' beneath you, making it possible for you to basically stand on your skis, on the water. Once you get going and find yourself advancing in a relatively straight line, you slowly add pressure to one foot, and begin to slide your other ski off by lifting your heel. More often than not, you will fall once your weight leaves the ski you are trying to get rid of. But eventually I succeeded in keeping upright after dropping a ski! Next summer's task: getting up on only one ski to begin with!