Sheikh Sa`ûd al-Funaysân, former professor at al-Imâm University
If the bathing costume that you are required to wear as a lifeguard reveals your most private areas (al-`awrah al-mughallazah) then you may not engage in that work.
If the bathing dress that you are required to wear as a lifeguard reveals the non-emphatically private area of a man (al-`awrah al-mukhaffafah) like the thighs for instance, then it would better for you not to engage in this work, but it is not prohibited for you to do so.
You should, if you are permitted, cover your `awrah in the entirety, from the navel to the knee.
You must lower your gaze as much as practically possible from the exposed parts of others who are wearing skimpy bathing costumes. You should only maintain the observance that you must maintain to carry out your job effectively.
If you are doing this work to earn a livelihood and cannot find other work, then it is permissible for you without any objection, since necessity requires it. In Islamic Law there is the principle that necessity makes impermissible things permissible. There is also the principle that need takes the ruling of necessity.
And Allah knows best.
Source: Islam Today