"A long-lived Magikarp is able to utilize its immense splashing power to leap high enough to scale mountains. (...) However, it is usually overlooked by Trainers because of its perceived weakness: even in the heat of battle it will do nothing but flop around. It is not a strong swimmer, and currents in the water will sweep it downstream."
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bulbapedia.bulbagarden.net/wik…So yeah... Apparently it IS a strong fish, just not for swimming xD