It seems counterintuitive but volcanos are most likely the source of earth’s atmosphere and much of the water along with carbonaceous chondrite meteors during the great bombardment. They are also what makes for catastrophic explosions in felsic eruptions. How to rehydrate Mars surface, I haven’t a clue unless the atmosphere is restored. Volcanism on Mars is long past. Any surface water except at the poles is lost to the thin atmosphere now.
With less gravity than earth, gases escape more quickly from Mars atmosphere. That is why there is not much. If a lot could be created, does anyone know how rapidly it would escape?