This question comes up quite a bit. The height at which you can fly at is limited by your jet ski, your weight, and the hose. A standard 60 foot hose weighs over 360 pounds when fully charged with water. Add to that your weight plus 60 pounds for the weight of the hose and board. You also need to factor in the pressure loss due to friction as the water moves through the hose.
So it is highly unlikely to exceed 50 ft or so even with a longer hose and the most powerful jet ski. But even if any of those was not a factor, flying above 50 feet (15 meters) is very dangerous. Imagine if you’re at 50, 60, or 70 feet high and the engine of your jet ski stops or the pump fails. Even though it’s water you can still break bones.