Further to Mark's response, I should add that a big part of selecting a shooting position is to avoid the very possibility of dropping birds in the water where crocs may be present.
If the proximity of water is unavoidable, such as at the Harrison Dam wall, then some restraint is needed to only shoot at birds whose flight direction or trajectory would drop them on dry land. Many birds well within range are routinely passed up for this reason.
If a bird was to make it back over the water before dropping, then it is left to recycle its nutrients at Mother Nature's behest but always counted as part of the daily bag limit.