I recommend to copy and paste Jupiter's .txt file so you don't have any extra work. But if you are using a planet that has collision head to "TERRAIN_DATA" and find "collider" and set it to false.

Once that been completed scroll all the way down to "ORBIT_DATA" and set the parent to the planet you want the water to be added to, make the SemiMajorAxis 0 and eccentricity 0

Congratulations! You have water! Unfortunately it has no psychics though, but it's still cool nonetheless in my opinion. But good news! With the 1.6 update we will get water! But bad news... we don't know when it will release.