The background is water. This is part of a series of photos of Zuikaku's sinking. As you can see from this photo of the last air attacks, she had taken on a list to port. According to the Zuikaku TROM, her list was 20 degrees by the end of the air strikes, and only increased. This famous photo facing the carrier's rear and this photo gives a sense of the list's severity. The photographer was likely photographing the scene from the Zuikaku's island, which gives a slightly more skewed picture of the list, but the list was quite severe.