Obviously, if you lived in the property, basic landlord tenant laws require hot water be furnished. Since this is a commercial property, you are generally not allowed to live there. If there is a need for personal sanitation (i.e. toilet facilities) a case might be made that hot water is needed. It sounds like this is an afterthought, and you have far less leverage than when you were in the negotiation stage. Maybe you can offer to split the cost of the water heater. The landlord's property is improved and when you leave, he keeps the water heater. You get hot water for half the cost you would otherwise pay.