My wife and I ski together without a 3rd and always wear the safety kill switch. Right before we jump off the swim deck before a set whoever is driving repeats the speed, letter, whether cruise control is on, and if the safety is attached (the kill switch). Every single time. No exceptions. Too many things can go wrong especially with no one else in the boat.
Most of us have heard the story of the driver who reached over the side of the boat to pick up a ski and accidentally hit the throttle and was thrown out of the boat. The boat circled both of them in the water and didn’t end well.
Even with a spotter or judge in the boat, I’m surprised the kill switch isn’t required.