system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we use salt water from the ocean for non-drinking purposes?

This was a question brought up to me by a seven year old, so I figure in order to give him a explanation he'll understand this might be the place. I'm kinda interested in the answers too.

I was talking about water shortages in places, and how there is a lack of fresh water, when the fact that the oceans cover so much of the Earth was brought up. Then the question came that if there is a shortage of water, why do we have clean water in our toilets, and why do we use clean water to wash cars rather than using salt water from the ocean? I'm assuming the salinity affects the surfaces of these things?
gpt
This kinda stuff was attempted in Hong Kong back in the 50's. They tried to promote using seawater to flush toilets, but it was unpopular due to the need to build a separate plumbing network for each house. In order to even try to implement what you're speaking of would require tons of money as freshwater and seawater would be coming in from 2 different sets of plumbing. One set for drinking and another set for everything else. The costs wouldn't stop at just homes though. Cities would need to also implement separate water supply systems just to carry the two types of water. 

Also, imagine all the salt and minerals in seawater. Imagine that building up in your pipes. It would lead to serious amounts of corrosion. Corrosion resistant metals rely on an oxide film as a means of protection. Think stainless steal or titanium. There are many ways to protect metals from corrosion, but that would mean completely new plumbing. No one wants to pay for that and it's simply incredibly expensive for no savings.