If they are anchored a ways from shore, then it would not matter if the water level goes up and down. You don't routinely let reservoirs get that low. When you

If they are anchored a ways from shore, then it would not matter if the water level goes up and down. You don't routinely let reservoirs get that low. When you have free power, you can pump water up to it. Desalinating, even, if necessary.