Was that said in response to telling someone to drink lots of water? or what were they referring to exactly? Without more context there is really no way of knowing.
Generally after a massage it is recommended that clients drink plenty of water to help flush out the body. Also helps to reduce the soreness in general, along with just being a healthy habit to get into. It's something that any good massage therapist would likely recommend.
Didn't really understand your question. You want to drink water after a massage to flush out toxins that's produced from the massage. If you are asking what to think of while having a massage... Then think of what the therapist is doing and how it feels. Tell the therapist things like, do that again. & it hurts there so could you work on that area a little longer? Or could you press lighter or harder. And when a sore spot is hit concentrate on breathing deep & slow.