The Latin plural is octopodes, which is acceptable in English, but octopuses (or even octopus) seems more at home in English. The form octopi is quite common in English, but it is pseudo-Latin. It is based on the mistaken belief that octopus was a second-declension noun like fungus. It has made its way into many English dictionaries, but I would not recommend getting tangled up with octopi. It is true that many standard English words have entered the language through mistakes (an apron from a napron, pea from pease, etc.), so octopi may not be totally indefensible. But people who know Latin, admittedly not a large group, will think less of you for using it.