Sherlock Holmes - Deductions

I walked slowly down the path. There was clay or soft marl mixed with the chalk, and every here and there I saw the same footstep, both ascending and descending. No one else had gone down to the beach by this track that morning. At one place I observed the print of an open hand with the fingers towards the incline. This could only mean that poor McPherson had fallen as he ascended. There were rounded depressions, too, which suggested that he had come down upon his knees more than once.
Once or twice as I hunted round amid the hard shingle I came on little patches of sand where the print of his canvas shoe, and also of his naked foot, could be seen. The latter fact proved that he had made all ready to bathe, though the towel indicated that he had not actually done so.

“Consider,” I said, “all the essential gaps in your case. On the morning of the crime he can surely prove an alibi. He had been with his scholars till the last moment, and within a few minutes of McPherson’s appearance he came upon us from behind. Then bear in mind the absolute impossibility that he could single-handed have inflicted this outrage upon a man quite as strong as himself. Finally, there is this question of the instrument with which these injuries were inflicted.”

“Surely it is evident that it is unequal in its intensity. There is a dot of extravasated blood here, and another there.”

Had the body been found in the water I could hardly have missed it. It was the towel which misled me. The poor fellow had never thought to dry himself, and so I in turn was led to believe that he had never been in the water. Why, then, should the attack of any water creature suggest itself to me?