I'm always a bit dubious when claims are made that this or that "requires" a certain amount of water to do. What kind of water, and what happens to it afterwards? It doesn't just vanish.

Water is just moved from a useful (to humans) state to a less useful state. The bottom of which is seawater. Seawater is still useful, it just takes more energy to do it. There's an unlimited supply of less useful water.

All that to say that once we use the easy water, that cotton shirt is just going to cost a lot more.