Another point is Chopper came from the a snow island, when it snowed the snow should melt in water and make them weak but he still could use his devil fruit.

Boa Hancock was a devil fruit eater and in the series she was taking a bath and luffy was there, she was standing up straight so was luffy, Hancock lower half of her body was soaked in water, plus the curse takes affect if your whole body is soaked