I've been told that to say that you want something is considered incredibly rude and childish. At least, in Japan it's supposed to be.
Japanese usually uses more indirect constructions to convey such ideas.
how should we say it in a more polite and indirect manner then?
I'd use it with friends but it would be a little blunt with strangers (just like"i want a glass of water" would be in english)
I didn't know MOTTO could be used with nouns - I'd have thought "more water" would be MOU MIZU, not MOTTO MIZU. Wrong?
Mou mizu ga hoshii desu. = I want water already. Neither motto nor mou modify the noun, but the verb or adjective. Throw in a number or such and you get that sense. Mizu ga mou ippai hoshii desu. Mizu ga mou nanpai ka hoshii desu.
Ima wakarimasu - doumo arigatou!
I learned 「水をもう一杯ください」 as the phrase to request another glass of water. What is the difference between もう and もうと to mean another/more?