The short answer is that "octopuses," "octopi," and "octopodes" are all fine (or so says Kory Stamper, an Associate Editor at Merriam-Webster).
But there's a longer answer, as well. One that involves a flow chart. And douche-canoes. Dear readers, I present to you The Darwin Eats Cake Guide to Pluralizing "Octopus," courtesy of my new favorite webcomic, Darwin Eats Cake:
Top photo by Mark Laita via Sea