Along with the juvenile flying gurnard, this interesting fish turned up during my recent blackwater dive in Palau:
It's a white-jawed cardinalfish (Pseudamia amblyuroptera), about 9-10cm in length. These fish are active at night, which probably explains why none of us recognised it at first.
The reason the photo is vertical is because that's how the fish was oriented. It was hanging there, vertical in the water, with not an apparent care in the world. The fish didn't seem bothered by lights or divers, and it didn't make any move to swim away. The most it did was rotate very slowly, just enough to make it difficult to take a photo.