A back-of-the-envelope calculation based on Newton’s Laws of Motion gives a figure of 20km/h. The lift force that keeps you up is a reaction to the force that you have to exert to move water out of the way as you travel through it. Its value depends on your speed, the area of ski in contact with the water, and the angle of attack of the skis with the water. From water-skiing photos and films it looks as though a typical angle of attack is around 15°, which gives a speed of 20km/h to just keep your weight up. Don’t take this figure too seriously though, as it doesn’t allow for frictional drag, bow wave effects or ski shape. It seems to fit with experience, however, and also with the claim that an elite K-4 kayaking team could tow a light waterskier at a record speed of just over 20km/h.