how about " give me some water "
that might be easier, anyhow, as the person who you are talking to or the person you would like him/her to do , just let him/ her know what you wish. that is enough.
The form of speaking english is not so important how correct you are, how official you are,
The most important is to let the person understand you, that is the most important
I would say turn on/off the water....its just me tho, I don't think this is too formal ...and I would say "turn the water up/down" for the second part. hope this helps