This octopus appeared to be walking on its "knuckles" toward me. An octopus is a very shy animal, but also curious, and extremely intelligent. They usually see you long before you see them. In fact, most of the time when I saw one in the daytime, it was when one was peeking over a rock, stretching out its body with its eyes looking out from behind a rock or coral head. When cornered, they have a remarkable ability to squeeze through the smallest of openings. This one stood about a foot tall as shown here, but could probably squeeze through an opening the size of a U.S. nickel coin.

When approached, if an octopus wants to get away its usually starts by crawling with its tentacles. If that's not good enough to get away it uses its siphon to squirt water for propulsion, in combination with an alternating opening-closing umbrella-like motion with the tentacles; and if that doesn't work the octopus will "ink" you with a cloud of dark ink, as a last resort. Octopuses also can instantly change the texture and color of their skin to blend with their background or show mood.

There is one species of octopus which is extremely venomous, called the blue-ringed octopus. It is a small species with distinct blue rings found in the western pacific, including Australia, New Guinea, Indonesia and the Philippines. At least two divers I knew in the Philippines have seen them there. Fortunately, they didn't try to handle them; their venom is known to have killed humans in as little as 20 minutes!

For more in-depth information I recommend The Cephalopod Page, a site by graduate student James B. Wood. His FAQ (frequently asked questions) page is especially useful.

Identification: Dare I guess?  too many species and their looks are always changing

The Cephalopod Page