Humans are not the only creatures on this Planet,
Scientists have described well over 1.7 million of the world's species of animals, plants, insects and algae. (Number of Species Identified on Earth)
Out of those, just imagine how many might be aquatic? That salt(ocean) water out there stands enough for the purpose to those marine inhabitants.
If you stop your question at...
why does Allah cover over 70% of the Earth with sea water?
This could be a possible explanation...
So we don't burn up in the atmosphere, the water cools down the planet. Technically there is more land than water anyway though since land is underneath the water. (Source)
Yes, everything is for a purpose, but not every purpose we may perceive.
Anyways, the creator knows that which we know not. And he creates as he wishes.
- Not a direct answer but an indirect one which I found from Surah Waqia'h (56).
In that after careful study of the verses 68-70, we find a logic behind why Allah sends down pure water which is helpful to us, humans. So, that we remember him and thank him for his help and mercy.
Tell Me! The water that you drink.
Is it you who cause it from the rainclouds to come down, or are We the Causer of it to come down?
If We willed, We verily could make it salt (and undrinkable), why then do you not give thanks (to Allah)? (Qur'an 56:68-70)
So, here Allah is threatening us, that he could make even the rain water salty, implying, thank him enough for what ever he gave you. If Allah would have made all the water drinkable or useful, then people would rarely remember him in terms of water scarcity.
- Regarding the ships sailing in the sea which others were pointing to, I found a verse regarding that...
Allah it is He Who has subjected to you the sea, that ships may sail through it by His Command, and that you may seek of His Bounty, and that you may be thankful, (Qur'an 45:12)
Again, the emphasize is that we may be thankful for letting the ships sail on that salty sea water. (I couldn't find scientific references yet for this)
May the creator guide us all.