One thing to make sure you do is talk to you kids about what happens if they fall off. My 6yo son is a pretty good dare devil IE: Enjoys snowmobiling, Jet Skiing, Skate park ramps etc. but he was still very nervous about what happens if he falls off for some reason. He is over it now but it was a real concern. Maybe you should "fall off" with her and get picked up to show here the routine. That boat looks pretty far away if your only 6!!!!
The other thing We did was keep the rope short at first but that was a mistake. The spray was too much... Just put it out 60-70 feet.