The old lady is too weak to walk by herself, so I decided to assist her.
She is too weak to walk, so I had to assist her.
Excuse me, sir, are you done with your meal? Can I take your plate away? <-- ‘are you done’ is informal, ‘have you finished’ would be more appropriate if it is not an informal situation.
Are you fine with your water? Or do you need a refill? <–your first sentence sounds extremely informal and I would avoid it as it sounds ambiguous (you appear to be asking whether the person is happy that they have water to drink). Your two separate sentences should be one sentence:
Do you have enough water or would you like a refill?
As there is no further context, it’s not possible to decide which tense is most appropriate, so I have accepted whatever suggestion was made in the sentence. There are other tenses which would work equally well in the right contexts.