plain water
Just plain water, thank you.

When you say something like this, do you mean "tap water"?

Question

plain water
Just plain water, thank you.

When you say something like this, do you mean "tap water"?

Answer

Yes, that's what I'd understand if I heard someone say this. The word 'just' reinforces the idea that you want ordinary water from the tap as opposed to bottled water.

Answer

Yes it's OK to use this real response.

In the UK, it would be inferred that you requested tap water and not bottled water. However,  most people would prefer to drink bottled water these days, and would ask for such.