This is what wikipedia says. So maybe I was wrong. Idk, after all Wikipedia is run by the Oompa Loompas.
"An urban legend has arisen around "In the Air Tonight". According to the legend, the lyrics are based on a tragic event Collins witnessed, usually a drowning (as in the song's lyrics), in which a man could have helped the victim, but did not do so (an alternate version of the legend has the onlooker a murderer, intentionally drowning the victim, rather than a bystander). A common ending is that Collins invites this man to a show and sings the song to him, often with a spotlight pointed at him. Afterward, the man is arrested or, in some versions, is wracked with guilt and has a heart attack or commits suicide. Other versions of the story involve Collins discovering his wife having s** with (or being raped by) another man, often a close friend; the man subsequently drowns while Collins himself watches.
Years later, Collins commented on the legends about the song in a bbc World Service interview:.
" I don't know what this song is about. When I was writing this I was going through a divorce. And the only thing I can say about it is that it's obviously in anger. It's the angry side, or the bitter side of a separation. So what makes it even more comical is when I hear these stories which started many years ago, particularly in America, of someone come up to me and say, 'Did you really see someone drowning?' I said, 'No, wrong'. And then every time I go back to America the story gets Chinese whispers, it gets more and more elaborate. It's so frustrating, 'cos this is one song out of all the songs probably that I've ever written that I really don't know what it's about, you know. "