νερό is water.
We do not really count water, so for the sea you would say: Η θάλασσα έχει πάρα πολύ νερό = There is very much water in the sea,
The plural is νερά, so you would say : δώσε με δύο νερά = give me two glasses (or bottles) of water.
But we do this only because make the association νερό = ποτήρι = glass of water, and we can count them with ease :
δύο νερά = δύο ποτήρια νερό = two glasses of water.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.