"Splash" has traditionally been a move of total disappointment, hence why people associate it with Magikarp. Splash usually does absolutely nothing. Well, not in Sun and Moon.
Sun and Moon introduces something called "Z-Moves", which are high-powered moves that can only be used once per battle, provided you have the right item. As the official site describes it:
There are two conditions for performing Z-Moves: a Pokemon must learn a move of the same type as a Z-Crystal, and it must be holding the corresponding Z-Crystal.
Data miners have found that Z-Moves can be paired with normal moves for added effect -- and that includes Splash. As pro player pokemon_PhD showcases in the video below, a "Z-Splash" boosts a monster's attack a whopping three times in a single go:
little vid of Z-Splash in action just to give you a better idea how Z-status moves work. pic.twitter.com/OejBLuuGPp
— Mike S. (ΩDonut) (@pokemon_PhD) November 10, 2016
After performing the move, the game will still say "but nothing happened", but it's far from the truth. Instead, Splash mega-boosts Gyarados.
Elsewhere, players are already trying to theorycraft potential competitive uses for Splash with creatures like Poliwrath, Mimikyu and even legendaries like Solgaleo and Lunala. It's not likely that Splash will be sweeping next year's big Pokemon tournament or anything, but it's still funny to think that the most useless move in the history of the game can now be one of the scariest, in the right hands.
(The bad news is that Magikarp is still useless even with this change. Alas!)