"...You shall not stand by [the shedding of] your fellow's blood...." [Kedoshim 19:16]
The Baal Shem Tov taught that everything that a person sees is orchestrated by G-d as a specific message to him.
This is the inner significance of Rashi's comment to verse 16: The fact that you see someone whose life is in danger proves that "you are able to save him". For the fact that G-d allowed you to witness this event must surely be for a practical reason - namely that you, of all people, have the ability to save this person.
Likewise, if one sees a person "drowning" spiritually, it is a sign from Above that one has the ability to draw him back to the fountains of living Judaism.
[Based on Likutei Sichos Lubavitcher Rebbe]