First, it is called Earth, not earth. Earth with a capital ‘E’ is a name, a proper noun. Earth with an uncapitalized ‘e’ is a regular noun, not a name, and it is basically dirt or soil. Soil took the noun ‘earth’ after Earth the planet.
Secondly, it is only called Earth by those who have adopted the Germanic etymology.
Thirdly, Earth is not at all mostly water. It is mostly nickel-iron, silicates and mineral, and with a tiny proportion of water at the surface. You wouldn’t describe a car solely by its paint job; don’t describe Earth solely by one characteristic. (info from Quora)
