[zan]

That's good advice from the guy in the shop, but (as a slight aside from your original post) the difference that stands out most to me between those two shots is the angle of shooting and the background. One of the 'rules' of underwater shooting is to get as low down as you can, and always shoot upwards onto your subject so that you really separate it from the background. It looks like you've done this in a couple of your photos, but it might help you compositionally if you tried getting down to the same level as the plants, and chose your background before the subject. Get that right, and the rest is much easier.