Gemara

There is no guardian against Arayos.

Kidushin 80b (Mishnah): A man may not be secluded with two women, but a woman may be secluded with two men;

R. Shimon says, even one man may be secluded with two women. If his wife is with him; they may even sleep together in an inn, since his wife will guard him.

81a (Rav Yosef): If there is an opening to Reshus ha'Rabim, there is no problem of seclusion.

82a (Mishnah - R. Elazar): Even a man without a wife should not teach children.

We are concerned lest he be alone with a child's mother when she comes for him.

(Mishnah): Anyone whose profession involves women may not be secluded with women.

Rishonim

Rambam (15): We do not appoint even a trustworthy, Kosher man to guard a Chatzer in which there are women, even if he stands outside, for there is no guardian against Arayos.

Poskim

Shulchan Aruch (EH 22:15): We do not appoint even a trustworthy, Kosher man to guard a Chatzer in which there are women, even if he stands outside, for there is no guardian against Arayos.

Bach (Kuntres Acharon DH u'Mah): Rashi explains that the Mishnah forbids one whose profession involves women to be secluded even with three or four women. The Mishnah forbids a regular man to be secluded with two women, but he is permitted with three or four. The Rambam forbids one man to be secluded with even three or four women, even if his profession does not involve women.

Bach (9 DH Ein): According to Rashi, who permits one man with at least three women, why do we forbid a guard? We discuss a Stam Chatzer with women, which can have many women! I answer that we discuss one who guards by day and at night. Rashi agrees that there is a problem at night, lest he sin while some of them are sleeping. Rashi permits with three women only during the day. The Beis Yosef says that the Ra'avad distinguishes like this.

Shevet ha'Levi (5:205): This is difficult. The Beis Yosef brought from the Ra'avad that Chachamim permitted two men only during the day, but at night we require three, lest one sin while the other sleeps. If so, we permit three. However, Rashi forbids two and permits three. If you will say that even three are forbidden at night, for we are concerned lest more than one of them sleep, we forbid even more. If so, we cannot equate Rashi to the Ra'avad! One could say that with women (i.e. more than one woman), we descend one level (and require an extra woman). However, this is difficult.

Shevet ha'Levi: If not for the Bach, I would say that this is not a question of seclusion. Rather, it is a special decree because he is an overseer over them and they are under him and submissive to him. Therefore, Chachamim were concerned for sin. The Chelkas Mechokek and Beis Shmuel ask from a Kal va'Chomer. Rashi permits even seclusion, so all the more so here should be permitted! They answered that here is more stringent. According to them, it is not clear to me whether they decreed about two or three men, since this is not due to seclusion. The Aruch ha'Shulchan compares this to seclusion. I do not know his source.

Chelkas Mechokek (17 and Beis Shmuel 17): Even according to Rashi, who permits one man to be secluded with many women, as long as his profession does not involve women, if he is a fixed Shomer, it is forbidden even if he stands outside. This is unlike standard seclusion. Since he is appointed over them, this is worse than a profession that involves women.

Drishah (25): The Tur says that one without a wife should not teach children, for the mothers come to get them, and he will be incited. He teaches that even if the wives have husbands in the city, so there is no problem of seclusion, it is forbidden.

Gra (21 Likut): In the Yerushalmi (Kesuvos Sof 1:8), R. Yakov bar Acha said that R. Yehoshua holds that Zenus pursues Pesulim. (If one was Mezanah, we assume that it was with a Pasul.) R. Ze'ira rejected this, for a Beraisa says that we do not appoint even the greatest Chasid to be a guardian against Arayos.

Birkei Yosef (7): Even one whose profession involves women may deal with them in a house open to Reshus ha'Rabim. However, this is only during the day and the first half-hour of the night. After this, it is forbidden, unless it is in a place where people pass even at night.

Nechmad l'Mar'eh (3 Kesuvos 1:8 p.91:3, cited in Otzar ha'Poskim 4): Sha'ar Yosef (3) says so. An opening to Reshus ha'Rabim permits also a overseer or one appointed over women. This is even for a person of low morality.

Otzar ha'Poskim (2): Apei Zutrei says that Chelkas Mechokek and Beis Shmuel, who say that a Shomer is worse than a profession that involves women, forbid even if his wife is with him, just like we forbid such a person. Apei Zutrei disagrees. Since he merely guards them, but he is not involved with them, surely it is permitted if his wife is there. He brings a proof from the Birkei Yosef on Sa'if 8, who permits seclusion with women in a Chatzer in which a married couple lives. What is the couple's Heter? It is because she guards him. However, one could argue and say that a Shomer becomes familiar with the women, so it is forbidden even if his wife is there, lest he find a moment when he can fulfill his desire, since he is there constantly.

Aruch ha'Shulchan (13): It seems that even two men may not guard women. Inevitably, one of them will go away or sleep, and the other will be secluded with them. Perhaps we can permit three men. However, according to the Rambam in Sa'if 7, even three are forbidden. In practice, this requires investigation.

Igros Moshe (EH 4:62): It is not immodest for women to swim in a place where there is a male lifeguard. He does not intend for lewdness. He engages in his job. Seclusion does not apply to such a big group, even for immoral people. If he would engage in his desires, he could not properly see if anyone needed to be saved. If someone died, he would be punished like a murderer. However, one who fears Shamayim should not bathe there, and a Chacham should not let his wife bathe there, if possible, lest the lifeguard put his eyes on her and try to come to her house when her husband is not there, and entice or rape her. Even though this is a far concern, it is proper for a Chacham to be concerned for it.

Divrei Yatziv (EH 45): A male lifeguard for women is not considered one whose profession involves women. Rashi in Kidushin gave examples of smelters and combers. Women need them. They get a personal connection with each woman. This does not apply to a lifeguard. Each day new women come. He does not recognize them. Even if he would happen to talk much with one particular woman, this is not called a profession involving women, since he does not work for them, rather, for the city agency that hired him. The women do not need him like in Rashi's examples. Also, even if one man is forbidden with many women, when there are so many that he cannot sin with all of them, surely some would reveal the matter. However, Sefer ha'Makneh (Kidushin 82a Tosfos DH Lo) says that an immoral man is worse than a profession that involves women. Beis Moshe (EH 22:8) says oppositely. Even according to Sefer ha'Makneh, the Birkei Yosef permits if it is open to Reshus ha'Rabim. Therefore, I permit. Also, sometimes men come from the city agency to supervise. Even if he is secluded, since new women come each day, he is afraid, especially since the authorities would punish him. However, in any case it is immodest to bathe in front of the lifeguard. It is disgraceful for men to look at Benos Yisrael (Sanhedrin 20a). One version says that Chachamim would not let a man gaze at a single woman (even though doctors said that this could save his life - Sanhedrin 75a). All the more so one must beware if the man is immoral.

Note: I heard that ha'Gaon R. E. Falk Shilta was asked (in a place where one could assume that the lifeguard is a Nochri), and he permitted. Ha'Gaon R. S. Forst Shilta was asked (where it is likely that the lifeguard will be a Yisrael), and forbade due to "v'Lifnei Iver Lo Siten Michshol".

Shulchan Aruch (YD 267:19): A woman may not buy male slaves, even minors, due to suspicion.

Taz (7): We are concerned lest she be secluded with them. There is more concerned for an Eved Ivri than for an Eved Kena'ani, for a Yisrael would not reveal if she sinned with him (Bava Metzi'a 71a). Seemingly, for the same reason she should not hire a young male servant. However, we see in all congregations that esteemed women have (male) servants, especially those who do business. It seems that Chachamim decreed only about slaves, for she owns them. Also an Eved Ivri is very submissive to her. It is forbidden, lest he consent to be secluded with her. Also, every slave, even an Eved Ivri, does not have such a Chezkas Kashrus, since he was sold. A mere servant has Chezkas Kashrus. He is not indebted to her. He would not consent to be secluded with her. Everyone knows that they avoid seclusion, just like men who have business partnerships with even single women. All the more so it is permitted if she is married, for her husband guards her. In Bava Metzi'a 71a we forbid a widow to host a Talmid. It seems that the suspicion is only for a widow.

Shach (31): The Bach learns from here that she may not hire a male Yisrael servant, due to suspicion. He connotes that even a married woman is forbidden, for he said Stam 'woman', just like Chachamim forbade all women to teach children.

Shevet ha'Levi (ibid.): The Taz suggested that it should be forbidden for her to have a servant, for he is subservient to her. He concludes that it is forbidden only if he is very subservient. The Shach forbids even if he is slightly subservient. If he is not subservient at all, and she is not subservient to him, and there is no seclusion and he was not appointed a Shomer over women, just he guards the property in the Chatzer, I see no Safek. Likewise, the custom is to permit if he lives there with his wife. Apei Zutrei was unsure.