Oceans - 22 free documentaries about coral reefs.
Start with Biscayne: Coral Reefs and the Florida Keys (44 min, National Geographic), then Coral Reefs - Magic of the Indo-Pacific (51 min, Free Documentary). All 22 documentaries here are free to watch from their makers' official uploads. To read deeper, the top-rated book on coral reefs is The Great Barrier Reef by Iain McCalman.