In the event that said governing body of such city shall order the laying and construction of such water mains, they shall issue ten-year serial bonds to pay the cost thereof, and the board of county commissioners shall levy and pay over to the city, annual special assessments against all property included in said benefit district petitioning for said water service, sufficient to pay the annual interest and to retire said bonds on maturity of the same, in the same manner and to the same extent as special assessments for lateral sewers are now levied in said cities: Provided, however, That where the governing body of such city shall deem it advisable to lay a water main larger than six inches in diameter in such benefit district, the amount to be paid by such special assessment shall not be greater than would be the cost of a main six inches in diameter, and the remainder of the cost shall be paid from the water fund of the city.