"Octopi" is my exception to descriptivism... Yes, normally, whatever words people normally use, and whatever words people will understand easiest without risk of confusion are by definition the "correct" words. Except "octopi", it doesn't count.
But more seriously:
"Octopodes" is the etymologically-correct plural, as that's how it was pluralised in both Greek and Latin ("ὀκτώποδες" and "octopodēs", respectively), but it's hard to call it "correct" in English if people don't know what you're talking about when you say it. Best saved for responding to people who try to smugly correct people by claiming that "octopi" is etymologically-correct. Also, be sure you pronounce it correctly - before watching that Merriam-Webster clip that I see has already been linked to aways up the thread, I had been pronouncing it wrong for years.
"Octopi" is a hypercorrection, but strictly-speaking acceptable from a descriptivist standpoint, in that people will know what you mean when you say it, but if you try to claim it's more correct than "octopuses", or that it was "octopi" in Latin, then you're entirely wrong.
"Octopuses" is entirely fine, as it's the standard English way of pluralising words.