Most likely a river otter they do dive as you say this one did and they like to play mind games with you at that. I had one snatching fish out of a floating basket one year down in norhtern Alabama. He would slip that paw up from the bottom as pretty as you please. as far as gator or croc the other folks are right they do not up end to dive and if a beaver does it normally slaps his tail at the same time as a warning to other animals in his group.