Wetpixel member Udo van Dongen has started an interesting discussion in the forums on the decision to crop or not. He writes,
My personal philosophy is that you can do almost anything to an image when it comes to colours, contrast, removing dust, unwanted models/turtles etc., because it was your personal experience which you can express in photos in the way you want to.
But cropping is something different to me. When you do cropping you can make any composition you like. Personally I think that, as an underwater photographer, you should be able to make a proper composition during diving and not only during post-processing.