Like the waters of the seas, their bottoms present a diverse series of biotopes, for one at very different depths, hence well-lit or dependent on whatever nutrients float down. Some look like flooded deserts, where most life forms hide in the barren sand. Others rather resemble prairies, either due to actual sea-grass or to kelp, where wildlife can graze, hide and chase.
However fragile the microscopic organisms that build coral reefs, no natural structure on earth is larger then theirs, especially the Great Barrier Reef. They change conditions completely for many hosted species hiding or hunting in, on and around the coral, from algae to sharks, and even contribute to island-building and shifting currents. Their own seed is confided to the sea once a year, drifting even to different oceans and starting colonies on any surfaces, including wrecked ships.
Many species of land creatures found their way back to the seas, requiring another round of evolutionary adaptation. Some became full time sea-dwellers, like whales and dolphins, who outclass predator fishes. Others became amphibious, like Galapagos iguanas, or nest and bread on land, like penguin colonies, who as birds also gave up flying. On the other hand, various sea creatures with gills, like crabs and even some fishes, regularly seek food on land.