why were the three friends fed up of scenery

The men are tired of scenery because at the end of a long day all they want to do is find the lock and relax for the evening. Thye're tired of rowing and want to settle for the night.

We all wished, however, afterward that we had stopped at Penton Hook.  Three or four miles up stream is a trifle, early in the morning, but it is a weary pull at the end of a long day.  You take no interest in the scenery during these last few miles.  You do not chat and laugh.  Every half-mile you cover seems like two.  You can hardly believe you are only where you are, and you are convinced that the map must be wrong; and, when you have trudged along for what seems to you at least ten miles, and still the lock is not in sight, you begin to seriously fear that somebody must have sneaked it, and run off with it.