Our on our island in Belize my absolute favorite underwater creature is the octopus. We see them at low tide in our tide pool, and also when night snorkeling. Occasionally you will see the Common Octopus during the day, but that is rare. Mostly we see the Caribbean Reef Octopus, which is typically nocturnal.
Octopus can take many shapes, including the ability to squeeze through almost any size hole. The only hard thing on their body is their beak, which is quite small. The rest of them are all skin tissue, so they can fit almost anywhere by readjusting their shape to fit the space. It is wonderful to see an octopus ooze across the sea floor. I have also seen octopus turn into a Frisbee and spin away from us, into a hole in the coral.