I can be brief. The beauty of closed containment is that the water is recirculated, so only 1% needs to be made up again. This is usually from a well-water supply. It's an amount that is not a huge amount of water in the scale of economic activity and agriculture.
To put it in perspective, if you took the amount of water needed to have the same production on land as in the ocean, it would be about the same amount of water use as one pulp mill.