Nice (tow) rope handling photo. In addition to lag and drag I see longitudinal acceleration with a possibility for a radial throw out. Depending on how fast the kids in the tube wanna go, I guess.
It seems like the boat and tube are more or less traveling in line , longitudinally there in that photo. Tube going the same speed as the boat. Did the boat ever take a sharp corner? Did it throwout the tube Radially? Did the tube accelerate to a higher speed and start to overtake the slowing boat?