This is a rare find.  In another video that I made with my underwater fishing pole camera (see earlier post here), I (unknowingly) went right by an octopus (he'e in Hawaiian, also commonly called by the Japanese name tako here in Hawai'i) in a shallow Mokule'ia reef on O'ahu.  He/she blends in well but the suction cups on the upturned arm give it away.  This might be a day octopus (Octopus cyanea) but this is really just a guess.  Clicking on the image below will link you to the original video.

Even better, the octopus has caught a cowrie!  It is hard to see but as the camera passes over you can catch the spotted pattern of (probably) a reticulated cowrie (Mauritia maculifera) just behind and under the octopus. Here in Hawai'i, octopi love to hunt cowries (leho in Hawaiian) and the cowries grow to some of the largest sizes in the world!  However, large ones are now rare in the waters around O'ahu.  In the image below that octopus is out of focus in the foreground and the mottled pattern of the cowrie shell is visible just behind it.  (There is also a sea urchin which gives a size comparison.)

He'e love leho so much that they are used as a lure for catching tako.  You can read more about that in the links from here, here and here.