The water fund shall consist of all money received on account of the water plant owned and operated by the metropolitan utilities district for water service or otherwise, including a water tax for public fire protection purposes levied by the municipal authorities of each municipality forming such metropolitan utilities district or, in the case of a sanitary and improvement district or unincorporated area forming a part of the metropolitan utilities district but outside the limits of a municipality, by the board of county commissioners of the county in which the sanitary and improvement district or unincorporated area is located. Such tax shall be levied at the same time and in the same manner as other funds provided for municipal purposes or county purposes under the provisions of the charter of such municipality or municipalities or of the general laws in the case of a county or a sanitary and improvement district. The amount of the tax shall be certified to the municipal authorities or the county commissioners, as the case may be, by the board of directors of the metropolitan utilities district in time for the annual levy of taxes in each year. The gross amount of such tax shall not exceed the sum of five and two-tenths cents on each one hundred dollars upon the taxable value of all the taxable property in such utilities district, and it shall be mandatory upon such municipal authorities or county commissioners to levy same as provided in this section.