If a natural lake, that means it's connected to its watershed and I bet having saltwater feeding into fresh is a bad idea. additionally since it will be fed freshwater from its tributaries and rainfall, keeping it saltwater would be hard.
If a reservoir with somehow all run-off blocked off, which is unlikely, good luck convincing the state to not utilize it as an actual reservoir for freshwater supply.
So it's not a problem of wasting water, the problem is first figuring out how to contain all this saltwater from damaging local wildlife.
..after that the problem is still wasting water for such a thing. Again, water is a valuable resource, and the only reason they build reservoirs to begin with is to keep a supply of freshwater, fisheries just happen to develop well in them.
