This is really an annoying issue. As you both point out, there are many more sources of pollution and noise than model boaters. It’s not as though there would be hordes of IC users at any one time.
Where I used to live in Northern Scotland I used to run petrol boats on Findhorn By, which is part of the Moray Firth. As full size power boaters used the bay there was no restriction. Obviously this was salt water and tidal, so really only suitable for reasonably large boats.
I can’t wait to get back there some day.
Deano.