The Aquaticans are originally from a planet named Marr in the Orion system - the galaxy closest to the Milky Way. Planet Marr was destroyed by an asteroid 10,000 years ago, which is when the Marrians migrated to Earth and became known among themselves as Aquatica - cementing their new beginning under the ocean. Mankind had not yet evolved - so the Aquaticans could have theoretically populated Earth’s surface, if not for them being allergic to our atmosphere and needing seawater to survive. (it’s the equivalent of oxygen to them) Earth was the closest planet to the Marrians that had marine water, so it became their new home.