Corpse size has a lot to do with it. I wouldn't swim in even a large pool with a dead human in it (knowingly), but one dead fish or rodent or dozens of dead tadpoles or bugs? Not an issue.
Heck, most household swimming pools have dozens of dead bodies in them, but they're 99% insects.
The whole premise of this meme is a bit silly. If there was a corpse floating near the beach, I think most people might wait for the corpse to be removed, and perhaps even a reasonable cause of death to be determined, before entering the local area. The same is true for pools.
If you knew there was a dead person next door you might be a little uncomfortable, but could go about your day. If you knew there were 50 dead people next door you would need to get out of there.
The number is relevant, not just the proximity to the closest one.
Nobody panics when things go "according to plan." Even if the plan is horrifying! If, tomorrow, I tell you that, like, you'll walk through a graveyard, or a morgue, nobody panics, because it's all "part of the plan". But when I bring ONE corpse to a job interview, well then everyone loses their minds!
People are often uncomfortable in graveyards and, for example, would not want to walk through one at night when they would be willing to walk through a field.
The dirt does provide a sort of insulation however, as people would be more willing to walk through a graveyard than through a house that had the same density of corpses in the basement. It's the theoretical accessibility to the corpse that plays a factor here.
A lot of human survival is based on heuristics, if you can tell there's a corpse in something, you probably shouldn't drink or eat it... As a general rule of thumb
For large body of water since you're unaware of the corpse two kilometers away on the bottom, it's probably not an issue for you.
However, primal human heuristics are not calibrated correctly from modern media. There was the reservoir where somebody was caught on camera peeing into it, hundreds of millions of liters of water, and they decided to drain the entire thing to prevent the public concern. That's just a heuristic run amok
A lot of human survival is based on heuristics, if you can tell there's a corpse in something, you probably shouldn't drink or eat it... As a general rule of thumb
And this is why it's dangerous to drink ocean-water.
Also why you should drink lots of that delicious peepee pool-water
ETA: If you're having dinner with someone who dies in the middle of eating their food, you can safely finish their food, drink, and poisoned soup as long as they didn't die face-down in it.
This PSA brought to you by the Society of Selective Listeners
Also what is the intermixing of water two kilometers away, especially affected by currents (which I presume, without checking ofc bc this is the internet 😁, are more horizontal than vertical - thus would intermixing occur more readily on the horizontal but the fact that it's vertical distance mean... what really)? So yeah, it makes sense then that due to the unknown factors, the default would take over.
Similar to the water:piss ratio regarding (US?) swimming pools, insofar as the knowledge that the "nostalgic" smell of swimming pools is not the comforting presence of chlorine so many believe it to be, and is in fact the confirmation of a volume of piss in the water that is rapidly nearing the extent of said chlorine's capacity to neutralize (sapped also by ceaseless sunshine & innumerable contaminants hitching rides on patrons' oblivious meatsacs).
In short: if you smell "pool", someone(s) have pissed in it. A lot.