You can buy one-time use underwater cameras for between $5 and $10. Depending on how many exposures they have, though, it might be cheaper in the long run to spend $100 or so on a waterproof digital camera. That way, you won’t have to worry about whether your pictures are “good” or not – you can always delete those that don’t turn out well. Make sure you check the depth to which the camera is water resistant, though – I’m not sure how shallow your shallow reefs are. There are a lot of digital cameras that are not specifically for underwater photography that are waterproof to a degree, but if you want something specifically for underwater photography it seems like you’d have to be willing to spend at least a couple hundred.