I have never spent any time thinking about it. Whatever is in the dock, usually cheap dishwashing soap, diluted or not. Most dishwashing soaps are promoted as biodegradable.
If you do 30 sets per day, at a generous 10cc per skier, it would be 300cc per day in a 100,000+ m3 lake. 100,000 m3 is 100,000,000,000 cc. I would be more worried about the gas and oil the boat puts in the water.