I was following this 18-24 inch (45-60 cm) octopus for about ten minutes before it decided it no longer desired my company. An octopus will try bluffing or crawling away from a potential predator first, or if more desperate measures are needed, it uses its siphon to rapidly squirt jets of water for a quick escape. As a last resort, a store of "ink" is used to cloud the water, confusing the predator's sense of sight and smell. Often the octopus may first cloud the water and do a "disappearing act" with its jet propulsion, much like a stage magician.
In this case, the jet propulsion and ink defense were used simultaneously. I was panning the camera along with the subject at the instant the shutter released, managing to catch the defensive maneuver in progress.
genus/species identification: unknown