Yes it was but did he engage in night swimming?
Not deliberately, no, but as with many young men of that era, time meant little when the long summer holidays stretched out before them. As he only lived at the bottom of the hill going out of D'arcy to Tiptree, traffic, according to him, was non existent.
He has just confirmed that it was often dark when he got home, but more late evening, as opposed to night.