The only correct English plural of octopus is octopuses. All other forms are wrong.
Octopus comes to English from New Latin, a sixteenth-century revival of Latin, which derived the word from the Greek okto (eight) + pous (feet). A small minority of speakers thus use the classically-Greek plural, octopodes (rhymes with Hercules), but since the word is not Greek, that plural is somewhat nonsensical. Far more speakers enjoy the academic air of the second-declension Latin plural, octopi, but those speakers make the un-academic assumption that octōpūs takes the second declension, when, in fact, it takes the third, where the nominative plural is octōpodēs.
So should we accept octopodes after all? No. We speak English, not Greek or Latin or New Latin, and the English plural, octopuses, remains in all contexts the only correct plural for that smart, slippery cephalopod.