He didn't see it at first. He was just swimming along, as calm as could be, looking down at a Lionfish he had spotted. The shark was swimming in his direction, and getting pretty close. I realized he didn't see it, so I banged on my tank to get his attention. He responded slowly, lifting his head, until he caught a glimpse of the shark, which was now within 15 feet of him. His calm turned into shear terror as he scrambled back towards us. He wasn't in danger (the shark gave no indication that it even knew we were there), he was just surprised to find it so close to him. If only I had been ready with the camera to take a video. It would have been humorous to watch how quickly he went from complete calm to complete panic.
In the excitement of the whole situation, I almost forgot I had a camera with me. By the time I got it together I only had a few seconds to snap these poor quality shots. I can't quite tell from these pictures which of the two Hammerhead species it is. The rounded front edge of the "hammer" makes me think it is a Scalloped Hammerhead, but the curved back edge of the ventral fin is indicative of a Great Hammerhead.
We also saw a large female Hawksbill turtle. It was resting on a ledge on the wall of the drop off and swam out into "the blue" as we approached. About halfway through the dive I spotted another shark in the distance over the drop off, but it was too far to tell what it was (probably just a Reef Shark).