Why Allah filled the oceans with sea water and not with fresh water?

It is mentioned in Quran that everything in the universe has a purpose. So, why does Allah cover over 70% of the Earth with sea water that human beings can't consume unlike the fresh water that is helpful for humans? What is the hidden wisdom behind this? Is there anything related to this mentioned in Quran or Hadith? A logical or a scientific explanation would really help, thanks!