(Rescuing a drowning person)

The answer – and Allah is the One who grants success – is that the intellect judges that it is obligatory to rescue one who has fallen into mortal danger, whether drowning or other kinds of destruction – this in general. Allah, exalted is He, says: “And whoever saves one [life] – it is as if he had saved mankind entirely.” [al-Māʾidah:32]
The proof that sound reason obligates this is that you see rational people severely reproaching whoever is negligent in such a matter, blaming him to the utmost for his negligence, no longer deeming him worthy of companionship thereafter, and regarding him as deficient in manliness.
The sharʿī proof that obligates this is that Allah, exalted is He, has made iḥsān (kindness) obligatory towards people: towards the believers in general, and towards relatives, neighbors, companions and others in particular. He, exalted is He, says: “And do good; indeed, Allah loves the doers of good.” [al-Baqarah:195] and He, exalted is He, says: “And do good to parents, relatives, orphans, the needy, the near neighbor, the neighbor farther away, the companion at your side, the traveler, and those whom your right hands possess…” [al-Nisāʾ:36]
It is not part of iḥsān to leave a drowning person to drown, or one fallen into mortal danger to perish, while one is able to rescue him from drowning or destruction. This duty, which both reason and the Sharīʿah together obligate, is only obligatory on condition of being able to fulfill it. Allah, exalted is He, says: “Allah does not burden a soul beyond its capacity.” [al-Baqarah:286] and He, exalted is He, says: “So fear Allah as much as you are able.” [al-Taghābun:16]
On the basis of this, what appears to me is that if someone present with a drowning person fears for himself that he will drown – because he does not know how to swim, or only knows how to swim a little – then rescuing the drowning person is not obligatory upon him. One who does not know how to swim, or only knows a little, is certainly in danger of drowning; and a drowning person cannot be rescued except by one whose skill in swimming is very high. For such persons as we have mentioned, rescuing the drowning person is not obligatory; if it is not obligatory upon them, then the diyah is not obligatory upon either of them.
Those who say that the diyah is obligatory on such a person probably looked only at the outward appearance: the one present with the drowning person has, outwardly, been negligent when he does not attempt to rescue him. This ruling of negligence is only according to the outward appearance. If he excuses himself by saying that he feared for his own life and was unable, we may say: that claim is not accepted and is a rejected plea, for a person’s testimony in his own favor is not accepted. Thus, the claim of fearing drowning does not justify the apparent negligence.
This is what can be used to justify the saying of those who make the diyah obligatory upon one who is present when someone drowns. However, it would have been necessary that if the bystander establishes upright testimony that he does not know how to swim, then he should not be held liable for the diyah.
If it is said: How can the diyah be obligatory upon one who has not killed, neither by direct action nor by causation?
We say: They have here treated negligence in rescuing the drowning person as standing in the place of a causative act, and thus given it the same ruling. Perhaps that was in order to ensure that human lives are not allowed to be lost without compensation.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2