Why did narrator think it was …

There is never a dull movement while the girls are towing the boat because they are in constant motion. Unfortunately, the boat is not. The girls entertain the men and detain them by consistently stopping to look for clothing, getting off the boat, and putting random items away. They also supply comic relief in the fact that they really have no idea what they are doing.
"Of all experiences in connection with towing, the most exciting is belong towed by girls. It is a senstation that nobody ought to miss. It takes three girls to tow always; two hold the rope, and the other one runs round and round, and giggles. They generally begin by getting themselves tied up. They get the line round their legs, and have to sit down on the path and undo each other, and then twist it round their necks, and are nearly strangled. They fix it straight, however, at last, and start off at a run, pulling the boat along at quite a dangerous pace. At the end of a hundred yards they are naturally breathless, and suddenly stop, and all sit down on the grass and laugh, and your boat drifts out to mid-stream and turns round, before you know what has happened, or can get hold of a scull. then they stand up, and are suprised."