I am not comfortable with "discharge" since it is not an accurate translation.
I agree, but couldn't think of another word from which I can also use the active participle. I welcome any suggestions.
Also "if only you thank" may be understood as a condition for God making the water bitter. I suggest "So won't you thank [Me]?"
But that would be a translation of فهلا تشكرون؟ The semicolon serves the purpose of starting a new sentence that is closely related to the preceding one, so the meaning is:
"If We will, we would make it bitter. If only you thank, We wouldn't keep reminding you."