The context is we’re ordering at a restaurant. “Two ice water, please.” Can we say this informally and drop “…cups of…”? Can “ice water” be pluralise as in “two ice waters”?

If there’s only two of you, you can just say “we’ll both have water” Ice/cold water is the default in North America. You don’t really have to specify it

Yes, it's normal to specify just the item and not the container unless they sell it in multiple sizes. You don't need to say "**a plate of** steak" but if they have different sizes you may have to say "a small/large Pepsi".

“Two ice waters, please” is the correct way, yes. “Water” here is not a group noun, it is singular). So because there are two of them, the word gets pluralized.

Yes, whenever there is a standard portion you can imply it. You can ask for 3 poutines, 2 beers and 1 coffee.

I would say “two ice waters please”

That is correct! and it would be pluralized because "ice water" is transformed from a description of the cup into a countable noun in place of the cups.

the word for this is a "synecdoche", where a part of something represents the whole of it. See also coffee: The bean, the liquid, the container of liquid.

I agree with the two ice waters... although I would say "Two waters please"... ok, no I wouldn't... I would say "Two beers please" :D But I would never ask for a cup of water... it is a glass of water.

'ice waters' sounds weird to me (New Zealand) - I'd say 'two waters with ice'. Ice water could just be really cold water with no ice imo.

Yes "a water" or "two waters" are fine because "cup(s) of" is implied

If you're ordering in the US, you can just say "two waters".

yes but let me help you out a little bit, can i get 2 waters. the ice is typically implied and if they arent sure if you want ice or not theyll ask. ive never been brought water without ice unless they asked me and i said no ice

Either "two iced waters" or "two glasses of iced water". I've never seen it served in cups.

Sounds fine. One thing I have learned though is ordering water varies by country.

Do you even have to say ice? I just ask for water and they automatically put the ice in.

Similarly, you can also say "two iced teas," "two iced coffees," etc.

Yes, you can definitely say that. But waters has to be plural, because you’re asking for 2 of them