I was looking at a small, unremarkable fish through my mask when, in front of it, I noticed a soft orange-y tube that was sticking out at a right-angle from a bell-shaped rock. After some seconds I concluded the tube was dilating and the rock had eyes, which were looking at me. I pointed at it maniacally, and it started to rise up, slowly, as if bracing for a dash. It kept its tentacles hidden to maintain plausible deniability.

I surfaced to tell my girlfriend. She thought I was pointing at the small fish. We went back under the water to look at it again, but it had gone. No witnesses, she had to take my word for it.

Note: I have since identified the orange-tube as its "funnel", and the octopus itself as a member of the "vulgaris" species.