Corals are the best known for fluorescing, but fish, plants, crustaceans, worms...here's one researcher's compiled list. many different kinds of organisms in many kinds of environments. You can find them everywhere! For instance, Malaysia. For an interesting overview of the history of human detection of fluorescence and underwater organisms, click here.
Well, researchers use fluorescent light to detect and study full coral reefs and juvenile corals that are too small (5mm in diameter) to be seen with visible light. Since coral reefs are some of the most diverse and most threatened underwater ecosystems, fluorescence is a useful way to do important research.