Good analogy with the water skiier! Air or water it's the same principle if you don't generate enough lift you sink. The hull of a boat is not a good comparison since it's using boyancy (like a blimp). The skiier is similar in that he/she needs some amount of force (water pushing against the bottome of the ski) to stay above the water and ski. If that force falls below the weight of the skier they sink. Same as in an airplance except the wing generates some lift which a ski does not.

Jeff