Took me a little while to find this but your story reminded me of something that happened to a friend of mine.
He came up on a dead Black Seabass just like yours while fishing his skiff at Catalina a while back. All that was left was the head half. They figured a Mako got it, but figured the Mako had left so they picked up the head half to get some pics of it. When they pulled it out of the water the Mako showed up, so they tied a rope on it and took some pics of the mako feeding on it, then cut it loose so she could finish her meal.
Adult Makos and Whites have been observed demonstrating territorial behavior when it comes to their kills. Like I said I bet a Mako killed that fish but considering how fresh your seabass looks it also wouldn't surprise me if that big Mako was still watching it from down below where you couldn't see her while you were paddling around taking pictures of it.
Jim