If you ever really want to get away from it all and see something that you have never seen, and have an excellent chance of seeing something no one has ever seen, get in a sub. You climb in, seal the hatch, turn on a little oxygen, turn on the scrubber, which removes the CO2 in the air you breathe, and they chuck you overboard. Down you go.
If you're going to make a lot of films about a particular group of animals, you might as well pick one that's fairly common. And octopus are: they live in all the oceans. They also live deep. And I can't say octopus are responsible for my really strong interest in getting in subs and going deep, but whatever the case, I like that.