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I think water should be weak to poison desu, mainly due to the fact water as a defensive and a offensive presence has complete control over its two weaknesses E.G. Every water under the sun has ice beam, and a number of relevant water types have a secondary ground typing or have access to earthquake to deal with electric types, giving it an advantage no other type can really abuse in a similar fashion.
This, compounded with the number of positive effects held by rain for a number of waters supporters or other coverage moves allows for a control that water can use on the offensive and defensive.
I'd suggest giving water a poison weakness due to fitting with the idea of pollutants (such as oil) being damaging to the coastal environment, but also meaning that an extra coverage option is availabe that water may find unconventional in being able to fully deflect like it usually can, though this is still kept in check by the number of psychic/ground water types, though now it cannot be as defensively controlling as before.
