the way i see it, an underexposed shot is more useful than an overexposed shot - post edit does miracles. what you should be more worried about are the colors. the deeper you go, the darker the blue gets - and increasing cyan and magenta helps.
however, if you are a purist, two stops, as mentioned by dan, is still workable. you might want to play around with your settings when you are still on the water surface before you go deeper. needless to say, overcompensate.