I hoped you saved before approaching the boss level.

No, this is not an amazingly hi-res preview of a boss enemy in a new Metroid game, it (terrifyingly) is real life. This video was taken by snorkeler Jonathan Gordon, who wrote,

This guy took me completely by surprise while snorkeling in the Caribbean. I dove down to have a look at the shell that you can see just under where the octopus appears and as I approached the octopus came out of hiding. I had literally no idea he was there until I was about a metre away.
Sources: Jonathan Gordon