In Three Men in a Boat by Jerome K. Jerome, what happens to the three friends' idea of bathing in the river? What was the writer's experience?

This incident occurs in Chapter XI of Three Men in a Boat. The friends had decided one night that the next day they would get up early and take a swim in the river. But when morning comes, George and Harris change their minds. Only the narrator, J., ventures along the shoreline, and steps onto a tree branch that hangs into the water. He’s having second thoughts, too. He means only to throw some water onto himself. But the wind is cold, and he decides instead to return to the boat.

and, as I turned, the silly branch gave way, and I and the towel went in together with a tremendous splash, and I was out mid-stream with a gallon of Thames water inside me before I knew what had happened.

The other two friends can hardly believe that he had the courage to go into the water. When they ask him how it is, he paints a wonderful picture of it. He can’t persuade them to try it themselves, however.