I have heard different values about this myself. And, the only thing I could gather was it depends upon your perspective.
For instance, water is water, so you should be able to drink water. However, what about the water in the oceans? That isn't just H2O. For example, there is also salt in that ocean. And, that water wouldn't be considered drinkable by many, even though some are trying to cure that with desalinization plants.
Or, what about the water in our bodies. For instance, even though I can drink water, I can't "get at the water" to drink in my brother's body. And, much of what we are is made of water. So, that water wouldn't be drinkable.
But, then, if you consider the water in the streams, natural streams, that water should be drinkable. Yes, unless a factory has been throwing its toxic pollutants into it.
In summary, the reason why so much water isn't drinkable is because of what that water is already combined with. We would have to separate the water in order to drink it. And, much of that we still can't affordably and efficiently do.