I understand and agree with you that we have wills, but not necessarily the power or ability to execute them. This is why we don't take the will to fly without the aid of technology or to see the future seriously. We know it's physically impossible. Therefore we filter our wills to fit within the parameters of reality as we understand it. This does not negate our ability to have these "wills", but that makes our "free will" essentially useless, as we're confined to play by rules decided externally.
You have to look at what determines our will. Let's use your prisoner example: As you said, your will is to have a sip of water. Why is this? You are thirsty or dehydrated. Did you will to be thirsty or dehydrated? Did you will your body to require H20? You are not in control of the circumstances and physical realities that determined your will. Your will doesn't just spring up out of nothingness, it's determined by your experience and situations you find yourself in. Another example: a guy is born with weak upper body strength, so he decides to go the gym to improve his muscles. His will is to improve his upper body muscles. His weak upper body strength is what determined his will, and he had absolutely no say in the matter of his upper body being weak. In reality, it seems to me your will is shaped by various factors that you have no control over. Who is in control of these things? It can only be the omnipotent creator. Subconsciously we realise the pointlessness of willing things that we are physically unable to do, so we disregard them. As mortal earth-dwellers, it leaves us within severely constraining parameters not of our own choosing, that play the main role in determining our will.He does NOT engineer Human will, but decides on whether or not to vest power into facilitating human action. Sometimes we succeed in our endeavours, and sometimes we don't. Our will is free and independent, but not "our" power.
So, on a day to day level, we are stuck in a situation where the factors that determine our will are beyond our control. Perhaps I should instead talk about the apparent uselessness of our "free will". It seems at odds with a fair and just God to me. "You are free to choose differently, but be aware of the sword hanging over your head if you do..." What point is there in giving us free wills, if you're going to punish us for exercising it?
If a person murders someone in cold blood, then God willed that person to be murdered, as the only way for the murderer's will to be turned into action was God granting him the ability to do so? Do you agree?Our free wills have not been predetermined. We decide what to will, and if God facilitates it, then we are accountable.
Is the guard an omnipotent deity that controls the physiology that made it necessary for you have to consume water?Take the following example: You are a prisoner, and your will is to have a sip of water. You tell the guard that you want water. The guard may grant or deny your request, and proceeds to grant you the water. You end up drinking the water. Whose will was done? If it was not your will to drink water, you would not be drinking it. If it was not the guard's will, you would also not be drinking it.
EDIT: To the people saying I'm exercising my free will by typing this: How do you know I'm not a robot that was programmed to do exactly this?