Based on a quick read of [1], it seems that using saline water is more expensive because of the preparation that is needed, so for now, it is easier to start with fresh water and add additives.

Also seems the consideration is about saline aquifers and not ocean water. I suspect the cost of transporting ocean water is the issue leading to this consideration.

[1] http://www.theglobeandmail.com/report-on-business/breakthrou...