Answer:
Hello!
Let's see which one will best answer the question.
- Mesera, la cuenta, por favor.
That would mean he wants the check, which you don't ask first
That isn't the answer, that would mean like "Great Job"
- Mesera, necesito la taza.
Since he already has a cup of water, the answer is
Mesera, necesito la servilleta.
Hope this helps!
-Fishylikeswater