Don't forget transmission costs too since not everywhere is by the coast. I live in a California coastal town that has spent 20 years just to come up with a project plan. They are currently doing an environmental review because it turns out that pumping concentrated brine back into the ocean can have detrimental effects. Read all about it at http://www.scwd2desal.org/
There are other problems too. For example my city has had excellent success in water conservation. However the fixed costs of the water infrastructure haven't changed so they have to keep increasing the water rates as consumption decreases, in order to maintain revenue covering the costs. (Arguably the fixed cost of the bill should be higher and the consumption charge should be lower to better reflect reality but that falls hardest on the poorer households and would discourage conservation by the larger consumers.)
I also recall reading somewhere about other unexpected consequences of conservation - sewage pipes had significantly reduced lifespans because effluent was more concentrated resulting in greater corrosion.
There are other problems too. For example my city has had excellent success in water conservation. However the fixed costs of the water infrastructure haven't changed so they have to keep increasing the water rates as consumption decreases, in order to maintain revenue covering the costs. (Arguably the fixed cost of the bill should be higher and the consumption charge should be lower to better reflect reality but that falls hardest on the poorer households and would discourage conservation by the larger consumers.)
I also recall reading somewhere about other unexpected consequences of conservation - sewage pipes had significantly reduced lifespans because effluent was more concentrated resulting in greater corrosion.