Agni and Water are man and woman. The havan kunda is the house. The result of the yajna is the progeny from their union. Hence,water is invariably placed by the side of Agni while yajna is performed. This water is placed to the north of the havan kunda because the natural sleeping position of the wife is to the left of the husband.
The water pot should be placed just at the border of the havan kund - neither far away nor too close. Nobody should go between them because it is inappropriate to go between a man and woman in union.