Given the agricultural issues in that movie, perhaps the purification device was intended more to collect and concentrate the urea than to purify the water.

But that assumes the scriptwriter would have thought of that, which may be a stretch, given that Earth has insufficient water to cover all landmasses to a depth of 7 km. That would require about 3.6 billion cubic kilometers more water than already exists in the oceans, and there are only about 1.4 billion cubic kilometers of water on Earth to begin with, . Certainly there isn't that much locked up in glacial ice. That is only 26.5 million km^3, which is enough to raise sea level by a maximum of 52 m, far short of the 8 km it would take to cover everything but Mt. Everest.

Anyone who would skip that bar-napkin math was probably only putting that scene in specifically for the gross-out factor. It's just like Mad Max eating the can of dog food. Which is an apples-to-apples comparison, because Waterworld was written specifically to be an ersatz Mad Max.