Will you please pass me the water?
Pass me the water, please.
Would you please pour me some water?
Would you please pour me a glass of water?
Excuse me. Will you please pass me the water?
We are more likely to use the word 'would' instead of 'could' because we are asking if the person is willing to do this. 'Could' asks if the person 'can' or is able, not if the person is willing.
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