A powerful point, but it relies on the idea that a single set of initial conditions can only result in one outcome. I'm not sure that's true?
Nobody knows if this is true or not, or even whether we could know this to be true or not.
For more info: http://plato.stanford.edu/entries/determinism-causal/
For example, I chose to ride a boat today, but it was raining, so I read a book instead.
That was what I was questioning, as the poster above noted, this isn't something you can assume. It's not really an argument for or against free will, just that particular explanation of why we don't have free will.