1. Driving the ski into something (another ski, boat, dock, etc.)
Skis don’t have brakes. Well some do, but these are only the most modern new skis. So using the throttle and steering to avoid collision is a critical basic skill.
Practice it. Teach it
2. Pulling a tuber and having the tube collide with something. (another ski, boat, dock, etc.)
Shore obstructions and other skis and boats are a big danger for tubers.
Be alert and be vigilant. Be safe!