The heart/lung shot is still your best option but as you know the recovery is still a challenge. For this reason I prefer late season, less and clearer water in the rivers and more crocs tend to bask longer giving you more options. These reptiles only need a couple of flicks of the tail to move a great distance after which you don't find them close to where they entered the water. They sink after the shot once they enter the water and unlike hippo they do not float after an hour or so.
As Jaco and yourself have said, small bodies of shallow water will be essential and hope that it does not have an underwater den. I have seen a 15 ft croc disappear in a water hole of 40 by 10 ft. If the water is clear and you do not recover it, it might help to cover the area by night with a small boat and spot light as you pick them up easier that way. It has helped me a couple of times.