"Get me some water," is abominably rude. Pour water on this child.
"I want more water," is rude. Do not give water to this child.
"I want more water, please." This is not polite. This is just an order with a please tacked on to soften the blow. Do NOT give water to this child.
"I'm thirsty," is merely information. Do not give water until asked nicely. What are you, the maid?
"May I have some water, please?" Ah... much better. This
child may have some water.