Hello, I experienced the same blossom of red bumps on the back of both of my hands. I know where mine came from. It was on Santa Rosa Wall in Cozumel. I was taking a photo of a lobster tucked into a nook, as I framed the shot in my viewfinder I felt a pretty strong stinging on the back of my hands. I looked over my camera and realized I had come extremely close to the lobster and some black superfine feathery life form surrounding the nook had rejected my advance.
I swam on for another 30 minutes and tried to put it out of my mind. When we returned to the boat I asked Sandro (DM) what it may have been. I had taken a picture which he reviewed. He called it Hydrocoral. Whatever it was it did not really bother me that much. Many small red bumps formed over the next 3 days and itched a bit but was not too obnoxious. We returnrd to Minnesota and it seemed to grow for the next 2 weeks or so. My doctor prescribed some steroid cream which did not do much. It seriously took about a month and then started to just fade away. I agree that my exposure made the back of my hands look like something out of a horror movie during that month. All better now and a little wiser!