When we see films or photos of underwater environments we always try to guess the location. We're pretty good at guessing by now, probably because we have dived in so many different areas of the world. We might not be able to guess precisely where a photo was shot, but we certainly can tell the difference between, say, a Caribbean reef and one in the Red Sea, almost at first glance.
For some reason, our ability to do this amazes many non-divers. There's really no trick involved. It's just that each region of the world has a characteristic array of marine life, and as a result, reefs in each region have a distinct 'look'.
This fact surprises many people, but I've never figured out why it should. I think it's safe to say that most people would not expect to see a tropical rain forest in England, nor would they expect to see moors near the headwaters of the Amazon. They know they would not see a giraffe in the wild in Montana, nor would they see a grizzly bear in Kenya. Similarly, what we see on Caribbean reefs is different from what we see in the Red Sea, and what we see here in Hawaii is different from both of those.
The first photo on this page is instantly recognizable as a Caribbean reefscape to any diver who has visited reefs in that region. The giveaway is those purple sea whips, so characteristic of Caribbean reefs.
The second photo on this page was taken on a deep reef in the Red Sea. It, too, depicts scenery that is unmistakably Red Sea. The distinguishing features in this case are the preponderance of richly colorful soft corals (Dendronephthia sp.), and the swarm of those little fish that resemble goldfish. Those tiny fish (Pseudanthias squamipinnis) are ubiquitous on Red Sea reefs. (Click on the photo to view a larger version -- and see if you can spot Jerry swimming in the background.)
There are diverse environments under the sea, just as there are on land. Each of those environments is a habitat for its own assortment of creatures. Perhaps those who are amazed to learn that reef environments vary from one region to another just never stopped to think about it?