Endangering yourself to save another

Q: A female athlete was rowing in the Yarkon River in Tel Aviv, when her boat capsized and she nearly drowned. People saw and they were concerned about jumping into the polluted water of the Yarkon. In the end, someone did jump in and saved her but she is in critical condition. Is it permissible for a person to endanger himself to save another person?
A: It is a dispute between Achronim (later authorities). While the Torah does say "Do not stand idly by your fellow's blood" (Vayikra 19:16), the Radvaz writes that one only needs to save him without endangering yourself as is known: "Your life takes precedence over your fellow's life." This is based on the Gemara in Baba Metzia (62a) which discusses the case of two people travelling in the desert, and only one of them has a jug of water. If both drink, both will die. If one drinks, he will make it to civilization. The Halachah is that one person drinks, i.e. a person need not save another people while endangering his own life. This is all-the-more-so true when one has the water, he is not obligated to give it to the other person. The Radvaz therefore said that it is certainly a mitzvah to save another person but one does not have to endanger himself since the ways of the Torah are pleasant (Shut Ha-Radvaz 3:625 and brought in Pitchei Teshuvah, Yoreh Deah 157:15). Other authorities disagree. They say that it is true that your life takes precedence over the life of your fellow, but it is obligatory for one to place himself in uncertain danger in order to save the victim from certain danger. This is the opinion of Hagahot Maimoniyot (Hilchot Rotzeach 1:14 brought in Beit Yosef, Choshen Mishpat 426) and Kesef Mishnah (ibid.). Their source is the Gemara in the Jerusalem Talmud that a Torah scholar was once taken captive. Many said: To our distress, prepare burial shrouds. Reish Lakish said: I am going to kill or be killed. Baruch Hashem, he was successful, but we see from here that Reish Lakish was ready to endanger himself to save another person. There is therefore a dispute.
In the article "Le-Mitzvah Ha-Aretz" (Le-Netivot Yisrael, p. 157), our Rabbi, Ha-Rav Tzvi Yehudah, agreed that a person should endanger himself to save another person. He brought a proof from the Gemara in Sanhedrin (73a) that when one who sees a person drowning in a river, or being dragged by an animal, or being pursued by bandits, he must save him, as it says: "Do not stand idly by your fellow's blood." In the majority of situations, a river, animal and bandit are dangerous. Furthermore, the Rambam (Hilchot Rotzeach 1:14) changed the word from "a person drowning in a river" to "a person drowning in a sea" which is even more dangerous situation. And the Rambam wrote (ibid.): "One who can save and does not save transgresses: Do not stand idly by your fellow's blood." A person who can save and does not obviously transgresses?! He means that the only exemption for a person not to save is when he is unable to save.
But in our case, jumping into the polluted waters of the Yarkon is a minor danger. If someone enters, he can immediately receive a shot which is like a retroactive immunization, and there will be no ill consequences. This is the responsum of Ha-Rav Ovadiah Yosef in Shut Yechaveh Daa't (3:84), to the question if it is permissible for a living person to donate a kidney. Ha-Rav Yosef holds like the Radvaz that a person does not have to endanger himself to save another person, but donating a kidney is a minor danger, and one does need to take such a risk. Thus, even according to the Radvaz, one needs to take a low level risk.
Therefore, they did have to jump into the Yarkon to save the athlete and "Yashar Koach" to the one who did. He is a national hero!