I was in your same situation two years ago. I went on a liveaboard on the GBR and figured I better bring back some photo evidence that I did it. I was able to rent a compact camera before setting sail from Cairnes. It was a decent compact with a built in flash with a diffuser. I got some amazing photos and absolutely fell in love with it. When I got home, I had to get my own, so I went with the Canon G7x with fantasea housing. From what I remember, this is the competing camera to the one you are looking at, so they are pretty close spec-wise. The housings usually come with the diffuser so you will be able to add light to the subject using the built-in flash as long as you are pretty close. Like Giffenk said, unless you are diving super shallow on a sunny day, you need a flash. As you descend, you lose colors from the visible spectrum (starting with red). Ive attached a link to a really cool video someone made demonstrating this (check out the red stick in the video). Anyway, I took it to the Caymans this winter, and the liveaboard let me borrow a strobe for the week. It made it just that much better. This year ill be adding a strobe to the set up....I sounds like we were thinking along the same lines, and I've been very happy with mine.