Ultratraditionalist is messed up too. The sun and moon were considered planets when geocentrism was king.
It comes from the fact that the word planet comes from the Greek and means wanderer. Any celestial body that wandered around compared to the stars was considered a planet.
We do. Titan is the only other body in the solar system with surface seas and lakes. They are made of hydrocarbons like ethane and methane however, not water.