The non-damaging and non-effect move Splash is a mistranslation. It's move name in Japan is はねる (haneru) which can be translated as both Splash and Hop (although usually as the latter). This mistranslation was mostly a result from the exclusive usage by Magikarp in Generation I.

This also explains why this move can be learned by non-aquatic Pokémon such as Hopip, Spoink, and Buneary for their nature of hopping and why it is disabled by the move Gravity.
Contributed by Naui