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Changes the target's type to mono-Water. This is not affected by the target's typing, or whether it is single- or dual-typed. Does not affect Pokemon with Multitype. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves. Effect lasts until target switches out.
Soak has the right idea behind becoming a competitively useful move; unfortunately, it is given out so sparsely that no Pokemon that gets it can make good use of it. Every Pokemon that gets it (with the exception of Azurill) is Water-type, and virtually none of these Pokemon have very good coverage. Perhaps Octillery could abuse Soak and Charge Beam to accumulate boosts to its Special Attack while taking out common counters, but this takes a bit too much time to set up to be efficient. Alternatively, Soak Azurill could be useful in Little Cup since most of its counters are Steel-types, and will not enjoy taking a hit from Azurill with that Steel typing gone.