What animals spend their whole lives in water? What are their features?

So, for example, clown fish spend their whole life in water. Their peculiarity is that they are all born males. And only then some of them become females capable of reproducing offspring. As a rule, females are many times larger than males.
You can meet these fish in the Pacific and Indian Oceans. Also, these inhabitants of the depths keep in pairs all their lives. In its natural environment, such a fish can live for about ten years, and if it is placed in an aquarium, then its lifespan can double.