Why did Charles feel he had reached the shore safely, even though he knew he was being followed? Why might the shark have attacked him anyway?

Charles was in water that was only 3 1/2 feet deep, so it probably seemed he could get away from whatever was following very quickly. It is unknown why the shark waited to attack. It was fairly young, so maybe it was unsure of what Charles was and finally, in the shallow water, decided it was its only chance to attack.