I will paste the reply I put in your other thread with the same question.
People do it successfully. Even the water you buy at Petco is natural seawater that they have packaged. But to do so, you have to try to avoid polution, parasites, so on. To do so, people collect their water at high tide. Once they do this, they run the water through UV
sterilizers for about a week before using it for water changes.
In my opinion, it is more work to get natural seawater going than it is to mix together some RO
and salt mix of my choice. That and then you can get increased calcium and such for a tank heavily stocked in corals.