For someone who wants/needs to drink there is less free will involved because the desire is strong. The brain wants something and so the reasoning can go both ways but the reasoning to continue wouldn't stand up to debate. Like when people often say "I can stop if I wanted to". If it is actually possible to stop then it is through us making a free choice because the want is powerful. This is why chemical dependencies of the brain are an interesting test for free will because if your dependent you don't really have free will, or do you?
You are free to follow your desires, whichever is greater, the pleasure received from the chemicals or the pleasure of being free from them.
However if you were denied access to the chemicals then your freewill would be impinged upon by an outside force.