Down island from me there is a public display aquarium, of sorts. It's called the Undersea Gardens, and it's a barge that floats in the harbour in Victoria. All the fish and marine life on display are native inhabitants of the area. In fact, it's their natural habitat, but it's been blocked off and protected, and you can view it from the lower levels of the barge, though large glass windows, giving the impression that you're looking into a huge aquarium.
They keep several Pacific Octopi in their 'dive tank'. Numerous times a day, a scuba diver swims in that tank and helps point out different marine life to spectators. He goes and finds an octopus and gently urges it to swim past the window so people can have a good look. It's quite a spectacular sight, as most octopus that I've ever seen in public aquariums are wedged into a rock crevasse or cave, and largely not visible.
I asked one of the ladies who worked there some questions about the octopus. She said they don't feed them; their environment within the tank is self sustaining (there are a billion crabs in there). She said they usually keep at least three, so that it's not the same one being disturbed constantly. They're intelligent enough to know different divers, even though they're totally covered in scuba gear. Apparently, some octopi like some divers better than others and one octopus likes one diver enough to come out voluntarily for the shows.
Sometimes they house octopi who've been injured in the wild and brought in by divers or fishers, as they stand a much better chance of survival and recovery in the protected environment of the Undersea Gardens. Otherwise, she said they only keep them for a couple of years (the Giant Pacific Octopus lives longer than 1 - 2 years; I want to say she told me about 8 years, but I'm not positive on that number), as after that, they fail to thrive, even though they're in thousands of gallons of water in their natural habitat. Surely, part of the problem is the disruption by the divers, but still, octopus are not well suited to life in captivity.
Just the same, I agree with all of you; having an octopus would be amazing.