DRAWBACKS TO USING ONLY THE INTERNAL FLASH
The length of the lens tunnel on the WP-DC28 AND WP-DC34 housings results in a dark area on the lower right portion of close-up macro photos when using only the camera's internal flash. The flash is partially obstructed by the housing lens tunnel due to is length. The included flash diffuser does not solve the problem. One solution is to back away from your subject then crop the photo in post-processing. Another solution is to place your subject in the upper portion of the monitor then crop the photo in post-processing. The best solution is to use a slave strobe. This problem is not as distinct in camera models with less zoom as the lens tunnel would be shorter. If you are buying an underwater camera and don't plan on using a slave strobe be sure to choose one that has the least amount of impairment using the internal flash.

In the below photo note the difference in lens tunnel lengths between the Canon S90 and the Canon G10 in their respective Canon housings. The S90 and S95 have pop-up flashes which may place the internal flash slightly higher than the fixed position flash on the G10, G11 and G12.

Below photo was taken with a G10 in a Canon WP-DC28 housing 4" from the subject in the Macro setting using the supplied housing diffuser. Note the darker areas in the lower portions of the photo.