Pause the video right away and try and locate the critter before it shows itself. Once you think you've found it, unpause and let the show begin. Let me know if you can find it.
Anyways, i thought it was a super neat video. The snorkeler apparently went down to pick up the seashell and ran into him instead which scared him. Fuck it would've scare me too if i seen him come out of nowhere like that lol.