They are tricky little bastards.
Your water has to be basically perfect - no small feat in a marine system. You need very high performance filtration because an octopus sweats ammonia, the scraps from its meals generate ammonia and ammonia kills octopuses. Nitrate will also kill octopuses. However, they don't seem to mind nitrite. (The tank cycle is ammonia - nitrate - nitrite, broken down by bacteria.) Your tank has to be absolutely escape-proof because an octopus can get through, and WILL get through, any opening large enough for its beak. (Yes, octopuses have beaks, and those beaks can be tiny - a 1/4" opening is enough to get any octopus you're likely to want to keep through it. They're also venomous.) The tank has to be huge and you can only keep one thing in it - corals will damage your octopus, any fish larger than your octopus will eat it, and any fish smaller than your octopus will be eaten by the octopus. You can't keep two octopuses in the same tank because they're cannibals. You decorate an octopus tank with stones, and the octopus will rearrange the stones on a regular basis. You have to hide your heater from the octopus because the octopus will take it apart - your best bet is to install a sump on your octopus tank, which is another tank that contains your heaters, filters and small fishes, and pump water from the octopus tank through your canister filters to the sump then back to the octopus tank. They're strong enough to push up the tank lid and escape, so it has to be bolted on. The prettiest octopus is the Blue Ring Octopus, which will attempt to bite you if you get a hand near the water and its venom is lethal to humans. Octopuses are very expensive animals to buy, and the ones that are suitable for aquarium life can live no more than a year. And how they die is pretty fucked up: a male octopus will just drop dead one night and leave you to find an octopus corpse at the bottom of the tank; a female octopus will lay eggs whether they're fertilized or not, then starve to death standing guard over them.
That's the bad part.
The good part is that octopuses are about the most fascinating animals you can keep in an aquarium.
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Maybe you could hitch some up together and use them as draft animals. 
