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

M:Then how about staying at my younger sisterís apartment? Itís near my apartment. She has gone on
winter vacation. If you come here, Iíll give you her apartment key.
W:Thank you so much, but shouldnít I ask for her permission?
M:Itís okay. She wonít care since she knows you so well. Oh, donít forget to turn off the main water when you leave.
W:Okay. Youíre my savior.