Thank you for adding the verse, it actually correlates to the amazing scientific process behind the complete water cycle. I already have upvoted the answer but I believe there is much to discover for a complete answer.

I am not fully satisfied with the answer as you haven't touched the water cycle. As per my little research into the subject, it is very much possible that initially all the water in the ocean was indeed fresh water which later turned into salty water because of the complete water cycle (water from ocean evaporates then falls on land as rain and then flows back to ocean with minerals and salts dissolved in it).

@Hakim No its not a duplicate. I am talking specifically about ancient people. Like immediate descendants of Hazrat Adam (AS) and Amma Hawa (RA).

Again to my original question, what about those who never had "The Thorah, The Bible, and The Quran" or any messenger at their times for guidance ?