the point of the water prison is to suspend the person so they are stuck in place. When kakashi was in the water prison, he could still look around and move his limbs, because its not a matter of water pressure that keeps him from doing so. However, like Shin_naruto said, when you're in a place where all the forces applied to you are equal, then you won't be able to move anywhere. Imagine drifting in space.....no matter how strong you are, you wont be able to control how you move around.....