Is it 2nd declension or 3rd declension? If it's 3rd, then you're right and it should be "octopuses". But -i is also the Latin plural ending for the 2nd declension, and given that -us is the usual masculine 2nd declension ending, "octopi" would make more sense.
But even then, Wikipedia says that it's from Greek "octopous" (which would get Latinized as "octopus"), which is "octo+pous" and "pous" has the irregular stem "pod-" which would make "octopodes" the correct plural. It also makes the guess that, if the word were truly Latin, you would have "octopes/octopedes" since "pes" is the Latin word for foot.
Also, anyone trying to claim that history/etymology has anything to do with the prescriptive "correctness" of modern language, go look up the etymology of the word "nice [http://etymonline.com/?term=nice]."
(second-year linguistics specialist, *****
)
EDIT: ***** not being directed at the author or anyone in particular, just added for effect 'cause I'm proud...