Sony F-717 digital, tripod
First off, I'll tell you that this little critter was slightly larger in size, but otherwise closely resembling, a paper dot from a hole punch. Very thin and remarkably flexible, it would typically swim through the water with fluid ripples of its sides in a manner similar to rays and ribbonfish. You're seeing all of the detail that I was ever able to make out, even at much higher magnification.
I think this is actually a larval or juvenile nudibranch, judging from a photo I saw some time back, but I'm still trying to locate a marine biologist who might know for sure. I caught these a couple of times in the lagoon.
At left, the dark areas are (I'm guessing) the back, and the pale 'arms' are actually the undersides of the flat creature, curled up in mid-ripple as it swam upwards. The central body seemed to have no appreciable increase in thickness, and the 'antlers' are just about the only thing that prevented it from being completely oval. In some photos a set of markings a short ways back from the antlers are visible, giving the appearance of a small mouth on the dorsal side but this seems, in detail, to be an impression only.
So, feel free to let me know.
Update January 2018: I'm going to correct myself, but again, only tentatively. After an encounter while out at Topsail Island, I'm leaning towards this being a juvenile sea hare – the appearance and behavior are pretty close matches, though the difference in size is appreciable. You know, the web is an amazing resource, but attempting to find some things still takes a bit of effort, like determining what exact terms one should be searching under...