The oceans are home to many kinds of animals. By "critters," we just mean animals besides fishes. The oceans are home to reptiles, mammals, and animals without backbones (invertebrates).
Reptiles are cold-blooded, air breathing animals. As you might expect, reptiles live in warm oceans. Oceanic reptiles include sea turtles and sea snakes.
Mammals are warm-blooded, air-breathing animals. Because their bodies heat their blood, oceanic mammals can live in the coldest oceans. Oceanic mammals also live in the warmer oceans. Oceanic mammals include whales and dolphins.
Invertebrates are animals without backbones. Invertebrates are the most numerous type of animals on planet earth. Invertebrates include corals, sponges, hydroids, worms, jellyfish, crabs, shrimp, lobsters, starfish, tunicates, and many other animals.