Pronounced octópodes, /ɒkˈtɒpədiːz/, with the -es rhyming with Heracles etc.

That's fun. Octopii rolls off the tongue though, doesn't it? Since we have survived both the Greek and Roman cultures, and have absorbed aspects of both into languages now widely distributed, I'd like to propose that we seed the path of a true lingua franca and declare the plural of octopus to be octopii.

It's no worse than inserting greek words (octopodes) into English language.