Why did the search for Montag veer inland?

This is because it would take all night to search the river; they would rather find a scapegoat to take Montag's place so all the viewers could see what they do to him. The viewers will think the scapegoat is Montag. 

The goverment wanted to make its people believe that they were in control and were strong enough to catch outlaws- and fast. Searching the river for too long would make people think that the goverment had lost Montag's trail and didnt know exactly what it was doing. They needed to ' save face' and appear in conrol of the situation. Also they had track of a man that they could substitute as Montag inland.