On a night dive, unless it's a full moon or you've stirred up a storm of bioluminescence  you can only see what's lit by the beam of your handheld light.  The exciting stuff is at the distant edges of that beam, where an animal might think you're just tossing around a little moonlight. Once they figure out you’re actually an enormous well-lit spaceship that sounds like Darth Vader, they’re outta there. But sometimes you get lucky- in that moment before you swim over a piece of coral, you notice its branches bend juust a little bit in the surge, it raises bumps on its skin, and it has eyes.  Before your brain can even process what you see, you freeze, hoping to recognize the cuttlefish before it recognizes you.  

For the animals that don't take flight--is it fright that keeps them there, or are they just sleeping?  An interesting paper on sleep and possible REM in cuttlefish:

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038125