Looks like debris but doesn't behave like debris

While snorkeling in Napili Beach in Maui last month, I saw a bit of debris that floated a bit oddly.  It moved to and fro in time with a current that simply didn't exist.  As I stared downwards through about 15 feet of water it entered a gap in the rocks, and ejected sediment.  An octopus!  It was the first time I've seen one in the wild.  I dove down to take a look.

The dark shape on the right. It's open siphon is near the top right. Likely a "day octopus" (Octopus cyanea)

In the absence of weights, I am so buoyant with a full breath of air that I have to expend large amounts of energy with my arms just to keep my head underwater. That makes my body crave air even more. To maximize my bottom time, I've realized it's more important to stay relaxed.  I keep only a third of air in my lungs prior to a dive, which makes me only marginally positively buoyant at the surface, and neutrally buoyant at 15 feet.  But with less stored air, it also means I can only stay underwater about 30 seconds or so (and 15 seconds or so is just to get down to that depth).

The beach that day (and probably most days) was crowded with sunbathers, and there were plenty of snorkelers in the water as well (this is one of the better snorkeling beaches, as it's sheltered by deep rock faces that attracting fish and turtles).  Despite all the snorkelers, few seemed interested in the octopus.  "Where is it?"  "I'll point it out," I offered, as I dove down pointing at the den.  A few seemed to stick around and politely look at what I pointed at for a minute, another stating "I don't see it", but all moving on.  It was, after all, probably too deep for most casual snorkelers.

After a while, I just watched from the surface, as it seemed more inclined to leave it's den when a human wasn't buzzing by with a camera outstretched.  And then, I noticed this:

It had an arm outstretched into a neighbouring crack.  A second octopus was in the other crack!

As with most solitary creatures, mating tends to be a cautious affair. Neither animal has the social experience to predict whether it will be hurt or eaten.  So mating tends to be an "as-far-as-possible" activity, with the male extending an arm to a female, positioning a sperm packet into the mantle (the head) of the female.  The mating I see in documentaries tends to have the octopus out in the open, with mating not lasting for more than a few minutes, so it was interesting to see them in dens going at it for over an hour (I'd hang out at the beach then revisit).  As for whose home turf this was, I didn't stick around long enough to find out.