I know sharks are a significant predator where I live and in places I’ve seen in documentaries. I’d assume that’s the case around all of the oceans since sharks and octopuses occur together everywhere. I’m not sure if that’s the case in the deep sea, though.

Probably not. I had found a few octopus in cetacean's stomachs, but they prefer squids. Gregarious and much more convenient to hunt.

Seals on the other time... Yup; I can tell you than monk seals definitely eat octopus.

The octopuses the author studied were shallow-water reef dwellers. They were predated by fish, mostly.