This is likely illegal disability-related (diabetes is a disability for this purpose) discrimination, which means your husband may be entiteld to compensation (or at least getting an order or settlement requiring them to allow him to hydrate). The law requires employers to make "reasonable accommodations" to employees with disabilities. Keeping a bottle of water at hand is reasonable: it is neither costly nor disruptive to the employer. Since it is reasonable, they cannot legally deny it; if they do, they face a discrimination claim. Your husband should speak to the federal EEOC or your state's equal/civil rights agency about this.