If your business is involved in food or some other business where cleanliness is regulated, contact the regulating agency. Otherwise, there is no law that can compell your employer to fix the bathroom.
The requirements of the Landlord to any retail tenant are governed by statute if it regards health issues and by the lease otherwise. If the lack of hot water creates a situation in which the tenant is unable to continue to operate the business (which does not sound like the case in this situation), the tenant may have an ability to vacate, stop paying rent and terminate the Lease under an argument of "constructive eviction". This is typically applicable in situations where it is impossible for the premises to be used for their intended purpose, such as when a roof leaks to the point that it is unsanitary or impossible to occupy the premises. Even in that case, the tenant must put the landlord on notice and give the landlord an opportunity to remedy the problem (in the case of emergent or urgent issues, the standard may be different). In this situation, as you have presented it, constructive eviction is unlikely to be applicable.
In your question you imply that the employer has some duty to provide hot water. I am not an employment law expert, but it seems that hot water is not necessary for a sanitary bathroom (it seems more important to have soap and toilet paper). If there was no heat in the winter, that might be different. Has this situation been addressed with the employer? Has the employer raised the issue with the Landlord?
As stated by the other answer, I would suggest you contact the local licenses and inspection and health departments and seek some help from them.
This response is not intended to provide legal advice or to create an attorney/client relationship.