@braindamage and @skialex yes I ski on Geneva lake or we should call it lac Léman....We have a slalom cours in Geneva and we have access through La Nautique from March to mid-November
http://www.nautique.ch/fr-ch/la-societe-nautique-de-geneve
There is no private slalom ski course in Switzerland because it is prohibited (don't know why..)
I think you might be able to ski at Cham on lake Zug, they have a waterski club but beside that we ski mostly in France nearby where we can have normal conditions which is not the case on the lake during summer time or windy conditions ..... to give you an idea, last year we had more than 35% cancellation due to weather. So from Zürich and the area, you have to go to Italy, or Austria maybe in Germany.
Go to https://waterskination.com/ and there is a map with a lot of places where you can ski and click on "find a waterski lake"
Good luck and if you need any infos .... don't hesitate to contact me through my email cholivet@gmail.com