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Ignoring the fact Kingler isn't useless just underused the fact is for the most part Ash tended to get one Water type a region and that Water type would be more marketable if it didn't evolve. It's when he caught a second Water type that this could be different hence Kingler and Palpitoad.
And while his other Water types aren't evolved they're still good. Squirtle is one of his trump cards, Totodile while mostly playful it had some decent wins including against Harrison's Sneasel, Corphish was very helpful in the Hoenn League and the Battle Frontier, Buizel was one of Ash's main battlers and still got wins after Infernape started stealing all the spotlight and Oshawott's biggest draw back was the fact it was the comic relief but when it got the chance to it could be a useful member of Ash's team.
And then you had Lapras which had no evolution line so it was basic and fully evolved.
