It seems like the issue could be that the
pond is not being operated as a "private ski lake."

It would be really tough for the government to come in and tell somebody that
they can't run their own private boat, even at a speed fast enough to waterski,
on a private lake solely on private property.

If one shoreline is owned by another party, it could get more problematic.
Noise could be an issue, depending on the proximity of any neighbors. Sort of
like the old rock band argument, one person's right to rehearse doesn't trump
the neighbor's right to reasonably quiet enjoyment of his home and property.

What is going on here seems to be that the owner of the lake is selling shares
in a
waterski "co-op" and inviting other people to come and use the lake. As such,
it could be argued that the private lake becomes a business and is probably
subject to the zoning and environmental requirements with which a business must
comply.