Drink mentioned in soorah Al-Mursalat is fresh water
2-4-2016 | IslamWeb
Question:
What are the drinks mentioned in Surah Al-Mursalaat?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and Messenger.
The drink mentioned in Soorah Al-Mursalat (chapter 77) is sweet, fresh water (maa'an furaata). Allaah, The Exalted, says while listing the manifestations of His Might and the blessings that He conferred upon His slaves, (what means): {Have We not made the earth a container of the living and the dead? And We placed therein lofty, firmly set mountains and have given you to drink sweet water.} [Quran 77:25-27] The reference to water in the verses is to delicious, fresh water that Allaah has caused to come down from the clouds or gush forth from the springs and run in the rivers of the earth for people to drink and water their animals, plants and trees.
As-Saʻdi 
commented on these verses, saying:
"The verses mean, 'Have We not conferred upon you Our blessings by making the earth suitable for life and a place of residence for you, serving your interests during your lives (houses and accommodations) or after death (graves). Allaah, the Exalted, bestowed the blessings of palaces and houses to reside therein in this world upon His slaves; He also conferred the blessing of graves upon them out of His mercy so that their bodies would not be devoured by beasts and the like. Allaah, The Exalted, says (what means): {And We placed therein lofty, firmly set mountains...} [Quran 77:27] This verse refers to the mountains with which Allaah made the earth firm; He set firm, huge mountains on earth so that it would not shake with people. He also says (what means): {...and have given you to drink sweet water.} [Quran 77:27] The verse means that Allaah bestowed the blessing of sweet, fresh water upon us; He says (what means): {And have you seen the water that you drink? Is it you who brought it down from the clouds, or is it We Who bring it down? If We willed, We could make it bitter, so why are you not grateful?} [Quran 56:68-70]."
Allaah knows best.