Translation:Squids and octopuses live in salt water.
'Octopi' is probably acceptable as being in standard use but is technically incorrect according to the Oxford dictionary. It's formed from a Greek rather than a Latin word which is why 'octopodes' is a correct form.
I posted a comment here yesterday but cannot now find it so I'll attempt to duplicate it. In Western Canada as well as in The USA's Pacific Northwest (in which areas I have lived for nearly one-half of my life) "Octopus" and "Squid" are each singular and plural. Each species is commercially important to the region and forms part of the diet of many people. As a consequence the two words are commonly used.
I don't recall hearing any of the words fveldig suggested as plural forms of "octopus" nor do I recall hearing "squids".
As suggested by MrsMBurns, the following words, in this region, are all plural: halibut, trout, salmon, cod, chinook.
Where in Norway does anyone use "akkarer" and "blekkspruter" rather than "akkar" and "blekksprut"? Those words sound weird to me. I would use "akkar" and "blekksprut" as both singular and plural.
Octopi is 100% incorrect because the word is not Latin, but a Latinised spelling of Greek Oktopous (pl. Oktopodês). However, whilst English occasionally uses borrowed forms for idiomatic and academic use (agendum/agenda, antenna/antenna), the use of Octopodes is incorrect because as soon as the word is borrowed into English, it must accept English grammar. Therefore Octopuses is the correct plural form. I'm not sure why this is still 'debated'.
Unfortunately the Oxford English Dictionary disagrees with you and gives three plural forms. Inflections: Plural octopuses, octopi, (rare) octopodes. Also the origin is Greek but the word comes into English through scientific Latin (1758 or earlier in Linnaeus), again according to the OED.
I still hold out a little hope that the English speaking and writing world will come to its senses and remove the mistaken back-formation, among others. Yes, descriptive linguistics wins the battle over prescriptive, but that also entails that we're all just as free to fight against changes as we are free to support them.
It's not a 'mistake'. Language is for communciation, and constantly evolves. Being a pointless pedant puritan doesn't help anyone. Language is heavily influenced by usage. So many people use it (and stadiums, and bacteria [for one bacterium] and all sorts of other stuff), it doesn't matter that it wasn't correct in Greek. Everyone knows what it means, and it's English.