The conservation of fresh water would be the "most energy-efficient way" to make sure the "planet has adequate water" for drinking.
Explanation:
Freezing ocean water, in the first place, would not be a good option as the ocean water would still be saline even after freezing it.
On the other hand, it would neither be feasible nor would it be economical to freeze the ocean water.
Though reverse osmosis and distillation are efficient methods of water treatment, it would not be possible to treat water enough to fulfill the drinking water need of the entire planet.
