Can a boss prohibit me from drinking water during working hours?

I suspect that there is more to the facts than you have offered here. Has the boss said that you cannot have water at your work station? That may be legitimate, depending on the physical set-up of the work station. Has the boss said you cannot leave your work station except for break times? That, too, may be legitimate, depending on the nature of your work assignment. Are bathroom visits also proscribed? Since you work in an at will employment state, meaning you can be fired for any reason other than unlawful bias (race, gender, etc.), it may not be practical to simply "stand on your rights" here. The better solution is to determine what is the particular kind of concern that the boss is attempting to address, and see if there isn't some middle ground that can satisfy those concerns without putting you in an uncomfortable physical situation. But, the bottom line is that there are many many jobs that preclude drinking water at will and, depending on the reasons and circumstances, that is not necessarily illegal so long as break intervals are in compliance with the law.

Your boss cannot legally prohibit you from drinking water at work. There may be an exception to this if having water present at your work space may cause contamination of some sort (as related to your job) or if it posses a serious danger. Otherwise, there appears to be no legitimate reason to restrict this.