main water

[keannu]

Doesn't it have to be "water main"? Or are the two interchangeable?

go2-9
M：Then how about staying at my younger sister’s apartment? It’s near my apartment. She has gone on
winter vacation. If you come here, I’ll give you her apartment key.
W：Thank you so much, but shouldn’t I ask for her permission?
M：It’s okay. She won’t care since she knows you so well. Oh, don’t forget to turn off the main water when you leave.
W：Okay. You’re my savior.

[Gillnetter]

No. Water main refers to a pipe and the valve which controls water flow. Main water, by itself, makes no sense.

[SlickVic9000]

(Not a Teacher)

I know what they're talking about in this exercise. Most houses (and I'm assuming apartments as well) have a main water pipe with a valve that can be shut off. I'm not a plumber, so I don't know if this can be called a water main, though technically a water main is simply "the principal pipe in a water distributing system of pipes".

[Raymott]

Doesn't it have to be "water main"? Or are the two interchangeable?

go2-9
M：Then how about staying at my younger sister’s apartment? It’s near my apartment. She has gone on
winter vacation. If you come here, I’ll give you her apartment key.
W：Thank you so much, but shouldn’t I ask for her permission?
M：It’s okay. She won’t care since she knows you so well. Oh, don’t forget to turn off the main water when you leave.
W：Okay. You’re my savior.

You can call it 'mains water', at least in AusE, not 'main water'. Mains water is water from the main city water supply, as opposed to from your own water tank, or elsewhere. You can also have mains gas and mains electricity.