Why and how did the three friends deceive the boatman?

The men deceived the boatman when they were trying to get away from the rain and abandon their trip altogether. They told the bargeman to care for the boat and have it ready for them the next morning, but in reality, they had no intention of returning. 

We had deceived the boatman at Pangbourne. We had not had the face to tell him that we were running away from the rain. We had left the boat, and all it contained, in his charge, with instructions that it was to be ready for us at nine the next morning. If, we said — if anything unforeseen should happen, preventing our return, we would write to him.