因为water=somebody.was=be动词.
be to do 表示将要做某事可以用,且表示1）按计划或安排要做的事 2）表示指令,相当于should,ought to,must,have to,This meicine is to be taken three times a day.3) 表示 想,打算,相当于intend to ,want to If you are to listen to the consert,you'd better hurry up.