Sounds like a great opportunity.
It also sounds like you've jumped to a conclusion that "public" means a full-time, unsecure, pay-per-use sports park.
Personally, I'd acknowledge their desire for the site to be 'public' and then work with/towards the most limited interpretation of 'public' possible, to start. "Public" may just mean they don't want to give you and your closest ski buddies an exclusive, restrictive lease. So, I'd start with something like:
Of course, I'm not looking for a private lease. Yes, let's make it open to the public.
But let's start modestly, with a pilot project to gauge interest, and figure out what access costs should be so that we're all adequately insured. Let's do a feasibility test with a water ski "club" - anyone can apply for membership, of course. And to save making a big capital outlay, let's start with members who can bring their own boat. For now, just to make sure we don't face shoreline soil erosion, let's limit it to USWA certified waterski boats. We'll get "club-based" insurance, but again, anyone can apply to be a club member, so it's still accessible to the public. We'll run a couple of "guest" evening events where non-boat-owners can come out and try waterskiing for a usage fee, and use that as a way to gauge how economically feasible it would be to (one day far, far into the future) maybe have a staffed, pay-per-use facility.
I'll also try to find a passionate SUP organizer who could apply to start a similar bring-you-own equipment, insured SUP club on another pit/pond. We could start working on a proposal to put a straight-line wakeboard cable park on another pit/pond, subject to determining economic feasibility. Of course, we would need some locked equipment storage (20' shipping container?) and a locked gate to protect the facility/equipment from vandalism.
Not hard to imagine getting you and a few ski buddies your semi-private paradise while maybe sharing the place with a couple of similarly-restrictive specialty clubs (if those other sports can get organized enough and meet yours-and-the-owners conditions).
"Open to the public" doesn't have to mean "open to anyone at any time with no conditions whatsoever".