And there is the problem. This needs to become highly UNUSUAL. like so unusual there are no known examples.
Under what authority does the government own rainwater? Do they also own the wind and the sunlight?
These people need to tell the government to go F%ck themselves.
Riparian rights are part of the Common Law inherited from England. They are part of the "Bundle of Rights" which a landowner acquires on purchase of the property.
If I were these landowners I would first look at the Title Insurance to see if this is an insured risk. If an attorney was used in the purchase and the intended use was disclosed to the attorney i would consider bringing an action against the attorney especially if they did the title work.
If all else fails and the lake level represents the surrounding water table level, consider drilling a shallow " domestic" water well for the double-wide and then use it for farm purposes.