Poseidon is the God of water, I think he understands how dilution works. Peeing in the ocean? Basically didn't do anything, essentially untraceable. Peeing in the pool? Nasty af, and easily(ish) detectable.
Unironically, why would anyone expect anything that happens after you die to be any less arbitrary and unpredictable than anything that happened while you were alive? Could be nothingness, but it could just as easily be Poseidon, or Flying Spaghetti Monster, or Kitty Cat Thanksgiving.
People love to attribute meaning and purpose to the randomness that occurs in life because the randomness can be scary, confusing, and frustrating. I don't blame them for that.
But perhaps if we all learned to properly cope with the difficulties in all our lives (rather than fool ourselves into thinking there is a mystical being controlling it all and judging us), we might be able to be nicer to each other.