As long as the legal maximum price is high enough that the seller can make a profit, the seller will change his behavior and sell the water for that price instead.

"Make a profit" is not the limiting factor. "Make enough of a profit to make it worth scouring the desert for thirsty people" is. If I'll get $3 for a glass of tap water, I'll sell it to someone who knocks on my door. If I'll get $100,000 for it, I'm going to rent a chopper and fly around the Sahara.