"Living in water" IS the definition. You are looking for a word to fit that definition. Aquatic is probably what you are looking for.
Animals that are adapted to living in water or on land.
What is the definition of "Living Space?"
A plant living with air with no direct connection to water or soil.
Living
alive and dead
The basic needs of living things are those things which are essential for that organism to survive, for example humans need food, water and shelter
The ocean is, as the French say, "flou." There is nothing exact about it. Its waters--note, you may as well write water--implicitly refer to a kind of lack of definition. What characterizes the living as such of course is definition, or boundaries. That exactly is not the case with the ocean and, by implication, the "being" "living" in it.
living; vital
The natural invironment that a living creature lives in.
No because it can't move nor does it grow or expand.
Operating or living or growing in water; "boats are aquatic vehicles"; "water lilies are aquatic plants"; "fish are aquatic animals".
Any environment that contain living organisms and food.