Thank you for the images John, they have created quite a buzz here. Please do post any screen captures when you make them in the future, we would very much like to see more.
I do believe that you are seeing the shadow of the croc, and I have a few educated guesses as to why you are not seeing the primary return.
1. Crocs are very faceted, and it is certainly possible that much of the impinging sound wave is dispersed by the textured skin of the gator. The features on a croc/gator are acoustically large compared to a 455kHz wavelength and would be excellent at scattering the sound wave, much like a stealth jet scatters radar pulses. I would not expect this to cause the croc to totally disappear, as there would still be a large percentage of directly reflected sound. This leads to point no. 2.
2. We implement time variable gain, where the signal is amplified more greatly as time increases from the transmit pulse. It is certainly possible that the croc is very close to your boat and the dispersion described in (1.) above is combining with the low gain at close distances to effectively erase the primary return. This would be an unusual circumstance; most objects even very close to the boat are imaged well, as those who observe fish in the water column will report.
An alternative explanation is that like you suggested, the croc is very close to the bottom and instead of directly reflecting the sonar pulse, his faceted features are scattering the sound, causing a returnless shadow at his location. In that case you would be correct that the shadowed area is in fact the primary target. The only reason that I don't fully subscribe to this explanation is that there is such a contrast between the bottom and the shadow, which I have never seen on a directly reflected return. If you look at most direct returns of textured objects like boulders there is less contrast among the features than your image shows.
Whatever the reason, it is a very curious occurrence and I would like to see any more pictures that you can come up with, especially showing a non-zoomed window.
Thank you again for taking the time to post these intriguing pictures.