Is denying water to employees at work illegal?

I work at a supermarket and they have recently re-instated a rule that says baggers/carriage people can't have water while they are working. i personally have a problem because these baggers have one hour shifts outside helping customers with their bags and retrieving carriages. on top of that the Company just changed our uniforms, they now have to wear BLACK shirts under heavy red aprons under a security vest (relatively lightweight ones). the new rule states that they can only have water when on break, however shouldn't they be allowed water when they're outside, seeing that we're hitting our summer months and it's already hot. just curious about this.