It's not a terribly rare occurence, especially in winter since super cold weather means it's easier to drown and there's generally less people outside in a position to notice a person in distress.

Since true crime content is a disease that causes brain rot, some people are convinced this is the work of a serial killer, despite the fact that there's zero evidence, occam's razor points to a simpler solution, police generally aren't in the business of downplaying / ignoring crime, and the fact that the next serial killer who wanted every death to look like an accident would be the first.

Hell, people willfully ignore that there's also incidents where people fall in the water while drunk and end up getting saved. One such instance happened like 6 weeks ago: https://www.nbcboston.com/news/local/18-year-old-leaving-celtics-game-hears-screams-for-help-helps-save-man-drowning-near-td-garden/2884563/