What is the ruling of Islamic law on one who hesitated to rescue a drowning person until he perished?

Saving a drowning person is obligatory upon the one who is capable and able to do so, provided there is no fear of perishing himself. He will receive a great reward for this, as Allah Almighty says: "And whoever saves a life - it is as though he had saved all mankind." (Al-Ma'idah: 32). So, whoever falls short in this despite being able to is sinful. However, the entire matter revolves around capability. If the rescuer fears perishing himself, then there is no obligation upon him.