This was a hard choice, since most of the photos I have have to deal with water in some way or another. I chose this photograph of the Great Barrier Reef because I thought it was beautiful and amazingly intricate. You can see so many different types of corals, as well as fish and even some clams. There are over 1200 species of hard corals known and the differences in morphology often translates to different growth rates. Branching corals tend to grow the fastest at 2 to 4 cm a year (but tend to break really easily), while dense, globular corals (brain corals) grow the slowest at 0.06 to 1.2 cm a year. Of course the growth rate depends on nutrient availability, temperature, light, and pH of the water (low pHs can dissolve coral skeletons).
Most people have heard of coral bleaching, the process where corals reject their algal symbiont and turn white. What few people know is that generally this is a natural process. If the water gets so hot that the algae living inside of the coral cannot function, the coral will reject that species of algae and try to obtain a species which does work in the warmer temperature. The issue comes in when the coral cannot get a higher temperature resistant algae or the water temperature rises well beyond what any coral-helping algae can handle. Then the corals die.