This relates to Evolution
For giving you understanding ; we need Free O2 but Oxygen is also available in H20 right why dont we use it !!
As the early atmosphere was reducing as there was no free oxygen but it was later produced by chemoautotrophic bacteria !! Living thing release O2 in atmosphere thus making it oxidising in nature and then life evolves !!
No because they live in the water so therefore they cant get thirsty lol
Because the gravity of the sun keeps all planets in their orbits :3 hope I helped