When I first saw this fish, at a distance, I thought it was a puffer, maybe a procupinefish, based on overall size, shape and where it was in the water column. But, when I examined my photo, it looked like a cowfish. When I zoomed in, I could not see horns. Perhaps, a filefish. Maybe a trunkfish, but the coloring is wrong. So I went to Humann's "Reef Fish." It looks like a (common) trunkfish, Lactophrys trigonus. Is it? How can I be sure?
Molasses Reef, buoy M2 (an offshore buoy), May 8, 2010, 52-feet deep, bottom temperature 81 degrees.