I disagree with the answer above. Most massage therapists are of the opinion that when you massage your body and drink water after you leave the massage table, toxins are flushed out of your system. The problem here is that no one can say which toxin gets flushed out and how the flushing out occurs. To several therapists, this is a vague theory, but to be on the safe side: apply precautionary measures like drinking lots of water after a massage. After all, drinking water never hurts anyone. Though over hydration which comes from drinking excess water can be a source of more problems that most people could ever imagine.
Offering post-massage water can be both polite and unpleasant, but no biological detoxifying need exists for this. It is just the same thing as following other health and fitness advice such as go for a walk more often to improve your blood circulation or eat more fruits and vegetables than processed foods. These are all beautiful but, tepid health advice.