In Outer Space there is no force of gravity. This condition is known as weightlessness. Many physical processes will happen very differently because there is no force that is pulling all objects in the same direction. Walking or lying down is impossible. If one drops an item, instead of falling down, it will float away.
Under the Sea there is gravity but there is also buyoant force acting opposite gravity. This force pushes an object out of the water. This means that the net force that pulls all objects down is reduced. Therefore, as an example, picking up a heavy item would be easier underwater.