>>31699896Pure water is better if you're a defensive Pokemon, true, but being a dual-type is nearly always an improvement for an offensive 'mon like OP's Basculin very clearly is.
Especially if said offensive 'mon has access to Adaptability, like Basculin does.
Dark also gives it an immunity, which means it can actually switch in on something now.
Really the only time where adding a type to a monotype makes it worse offensively is if the new type provides little new coverage while compounding existing weaknesses, like adding Grass to Fairy or Bug.
