Well that is an interesting shot. It certainly has some features of the head of a salty. So hints to help. What was the scale of the shot?, I.e. depth of water and range of beam? This will give some clue as to the size of the object and then if it looked right so and within the size you would expect for a croc. I did see there were some smaller crocs in Manton Dam but if this is a head it looks big. If you are also looking at what I am looking at and it is the head of a croc you are seeing, there is no body. Overall I think not. I have seen many crocs both freshies and salties and I always see the whole shape. I know that some large salties have been seen on the Daly without heads and possibly trophy hunters, so as you know, to see a salty without a head is more likely than seeing just the head.
If you would like some more shots of salties I can post more.