Octopodes is the correct plural. Octopi is incorrect...
Here's an accurate article:
Plural
A note on the plural form: Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses", and that octopi is misconceived and octopodes pedantic. Octopi derives from the mistaken notion that octopus is Latin. It is not. It is (Latinized) Greek, from oktopous (ὀκτώπους), gender masculine, whose plural is oktopodes (ὀκτώποδες). If the word were Latin, it would be octopes ('eight-foot') and the plural octopedes, analogous to centipedes and millipedes, as the plural form of pes ('foot') is pedes. In modern, informal Greek, it is called khtapodi (χταπόδι), gender neuter, with plural form khtapodia (χταπόδια).
That said, Merriam-Webster and other dictionaries accept octopi as a plural form. The Oxford English Dictionary lists octopuses, octopi, and octopodes (the order reflecting decreasing frequency of use), stating that the last form is rare. The term octopod (either plural octopods and octopodes can be found) is taken from the taxonomic order octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food. Finally worth mentioning is Octopussy, a blend word of octopus and pussycat, which found its way back from the movie title to a term of endearment for the animal that had originally inspired it.