I was on the same liveaboard as H2OGal and JDP, (Hi friends!!!) and their advice is sound. I want to emphasize having enough batteries and media cards. Also, bring your laptop with photo editing software, something easy like Picasa is great. It was wonderful to be able to see my shots at the end of each day. Many of us sat around the salon in the evenings admiring each other's shots. I brought a novel and never even took it out of the bag. I also brought a book about u/w photography and borrowed one from JDP but never did more than just look at the pictures.
On the other hand, a few folks on board had their nose stuck in paperback novels the entire time they were not in the water, including at mealtimes. Heck, they don't weigh much or take up much space so if you're an avid reader, bring 'em.