In the water or on land, it's the same. Have your camera ready whenever there's even a remote possibility of something interesting happen.
As the DEEP Indonesia show wound down, I stood outside the exit, just watching people. The light levels were low and the scenery generally chaotic, but I still managed to get a couple of nice photos.
I saw this situation developing, so I rushed over to snag this photo, much to the amusement of the guy's friends.
Besides anticipating the situation, I had everything preset on my camera for the ambient light conditions, so all I had to do was compose and shoot.
Basically, it's the same underwater. Anticipate the conditions, adjust your camera settings in advance, so you can (primarily) just worry about composing and capturing the moment.