The Reefs are found in the shallow waters, to get a sufficient amount of sunlight to survive.
The coral reefs need sunlight for survival because in the coral cells symbiotic lives and they need sunlight to carry the photosynthesis.
zooxanthellae live inside the Coral cells, they get protection and compound which is required for the photosynthesis from the coral and in return zooxanthellae produce oxygen and help the coral in waste removal and photosynthesis products glycerol, amino acids and glucose to coral reefs.
They need clear water, so that sunlight can easily pass through it.
