Splash is a Normal-typemove introduced in Generation I that has no effect. Why Gamefreak added such a move is unclear, although it may be to emphasize the weakness of Pokémon such as Magikarp or Feebas.
Despite being named Splash in English, it is not a Water-type move. This is due to a mistranslation of its Japanese name; Hop. This explains why such non-Watertype Pokémon like Hoppip and Cleffa can learn the move.