Most of the liquid water on Earth is found in the oceans, in fact the percentage is so big that it comes as if almost all liquid water on our planet is in the oceans while the water from the lakes, rivers, ponds etc. is just a minor part of the whole.
The water from the oceans contains salt, thus making it a salty water and not suitable for drinking as it is naturally. For this water to be consumed and be part of the daily of the humans for drinking it needs to be desalinated. Desalination is a process that gets the salt out of the water and makes it suitable for drinking. This is not an unknown or untested process, it is something that is practiced for decades now mostly on the Arabian Peninsula where there isn't any fresh water resources.