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If rain is the prevailing weather condition, the wielder heals itself of burn, freeze, paralysis, poison, and sleep at the end of each turn. If rain ends on the same turn that the wielder is afflicted with a status, that status will not be healed.
Hydration is an absolutely amazing ability in the OU environment where Drizzle Politoed is available. Being basically immune to status due to the permanent rain is an absolute godsend to any defensive Pokemon that gets it, particularly Vaporeon. Hydration is also what arguably pushed Manaphy over the edge, as inflicting it with status in order to outstall or cripple it as it set up was simply out of the question, and even if it got to low health it could use Rest to reach full health and then rid itself of its sleep on the same turn. However, in any metagame where permanent rain is unavailable, Hydration will usually be left out in favor of more useful abilities in common battle conditions especially because having only 8 turns of status immunity is far less useful than having something like the double Speed that Swift Swim brings under the same conditions.
Found in Dream World