Ohmigod, crazy mary again (aka dj, aka dj1).
The landlord can be the supplier of water and charge what he wants. If you want to go elsewhere for your water, go ahead. Don't forget that this is America. Its called deregulation.
Next, landlord charge all tenants the same amount for water no matter what the usage? Check Md law and also the lease.
As an example, CA law says If landlord does not provide separate meters for each tenant's dwelling unit so that each tenant's meter measures only the service to that dwelling unit, then landlord shall execute a mutual written agreement with the tenant for
payment by the tenant of the cost of the service.
Now what does MD law say? That the landlord cannot charge each tenant the same, regardless of usage? Check the lease? What does it say about payment for water? What does the law (and "mary") want the landlord to do?