The format of the media actually does not really matters. CF is bulky. SD is fragile. Both store zillions of pixels and do their job pretty well. However, don't think that all media within the same family come equal. Speed is of the essence. When you're shooting underwater (or anywhere else actually), it's important that your camera be available all the time because you don't know if that fishbutt you just shot will turn around and smile at you in a perfect great shot! If your camera is busy saving your last shot, it cannot take a new one and it's very very very frustrating (even more than the autofocus delay). So the solution is whatver the media format, choose the high speed high-end media. Sandisk ExtraII or UltraIII do the job pretty well. Remember that the more pixel per image you have, the faster you want your media to be. ExtraII may do the job for a 6Mpix camera but UltraIII will be needed for 8Mpix or more.