Translation:I want more water.
Does "hoshii desu" in Japanese sound as rough as it does in English or is it formal enough? (Would you use this structure in a restaurant instead of "mo chotto omizu o kudasai" for example?)
Surely ほしいです is a bit rough and childish. いただけますか is very polite expression to require something. (いただく is of いただきます(Japanese greeting? before eating a meal)) But be careful not to just replace ください with いただけますか. Because もっと is also childish expression, i feel もっと水をいただけますか is strange. I use おみずをもういっぱいいただけますか in restaurant. (もういっぱい one more cup of)
Hoshii feels like a demand, you wouldnt use this word with strangers or even acquaintances. Too rude.
They should really add "I would like" as an option for a lot of these asking questions.