@A Wild Bidoof, the only move Magikarp knows until like lvl 20 is splash, which does nothing. Rain doubles water-type pokemon's speed, so it does nothing twice as fast
@FlyingAcorn, actually magikarp learns tackle at level 15, but this is actually a reference to magikarps ability. Swiftswim: when it is raining this pokemons speed doubles.
Time to switch into MAXIMUM OVERFLAP