The mand “water” occurs as a function of some degree of water deprivation, which is the establishing operation for this response form. If you say “water” because there is a glass of water present and the form of the response is not controlled by water deprivation, then the response is not a mand. The glass of water in this case is a non-verbal stimulus and the response would, therefore, be a tact.
A form of verbal behavior with the following features:
The response is verbal
The form of the response is controlled by an establishing operation
The musculature with which the response is executed (vocal, written, gestural, etc.)
The type of reinforcement made currently effective by the establishing operation (may be conditioned or unconditioned, etc.)
Whether or not the current response is followed by reinforcement