This is correct, illegal to ask unrelated tenants to split a water bill. Not necessarily saying it’s a good idea to tell your landlord this, theyl probably ask you to move and rent to someone else that will accept the split bill. Unfortunately it’s difficult and most of the time prohibitively expensive to split services and add meters. The landlord should inflate rents to cover this cost, but I would guess it’s been like this for a while and you won’t get him to budge